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A prefabricated anti-tank pillbox – worthy of its own name?

Pillbox characterisation is nuanced. The adopted naming convention often refers to these hardened field defences from the Second World War to the drawing number issued by the War Office (WO) Directorate for Fortifications and Works (DFW); for example Type 22 or Type 26. The DFW branch responsible for defence design was branch 3, sometimes resulting in a design designation of FW3 followed by the pillbox type; FW3/22 or FW3/26 as examples. Design specifications were also issued depending on the level of protection offered; this could be bulletproof (also known as splinterproof) or shellproof. Most short references to pillboxes do not record this wall thickness as standard, although designations such as FW3/26 shellproof are often seen. The only exception to this could be when the Type 24 is viewed as a shellproof upgrade of the Type 22. It is this simplification that often leads to some of these nuanced features being missed.

There is a now over 10+ years old article on the Pillbox Study Group website entitled a new type of Pillbox. Based on extant naming conventions, this new type of pillbox would be a simple FW3/26; a small, square pillbox with three or four loopholes and a lowered entrance.1Spotter’s Guide to Pillboxes, Mike Osborne, 2020 So why the fuss? While the pillboxes did indeed match the specifications for a splinterproof Type 26 pillbox, they had been constructed from pre-fabricated components. After some research, it was discovered that the components were fabricated by the Stent Pre-cast Concrete Limited; so the Type 26 Stent pillbox became common nomenclature.

But what if other manufacturers were producing similarly kit-like defences? It’s possible that in the west of England there was a manufacturer doing just that.

A FW3/26 prefabricated or Stent pillbox at the National Shooting Ground, Bisley, Surrey.

There are of course pillboxes that utilise prefabricated panels that are not in themselves worthy of a new designation. This example, another FW3/26 splinterproof, has utilised prefabricated panels during the construction process, much in the way brick was often used. The panels aren’t bolted together, nor are they structural. They have been used as formwork when casting the concrete filling to expedite construction and reduce the reliance on scarce materials such as brick and wood. This example is along the River Severn at Arlingham.

New design or just time-saving shortcuts?

The pillboxes I am referring to have been constructed along the Stroudwater Navigation in Gloucestershire, in particular, I will use the surviving example at Upper Framilode to demonstrate what I believe to be a new type of prefabricated pillbox. Part of Stop Line Green, they were constructed in 1940 and formed a defensive ring around Bristol. A very comprehensive report was issued in 2007 by the Gloucestershire Society for Industrial Archaeology (link) which highlights the use of prefabricated panels in expediting the construction of these pillboxes.

The techniques used in the construction of largely prefabricated pillboxes would have provided considerable time and therefore cost-saving benefits to the construction company contracted with the work. The number of prefabricated parts that have been included in these pillboxes can only mean that industrial-scale production was in force, elevating their status from merely using prefabricated panels to being a form of flat-pack defence.

The image from IWM showing Royal Engineers constructing a pillbox demonstrates the extent of shuttering or wooden formwork required to cast a concrete pillbox. By utilising prefabricated sections much of this work can be reduced to hours instead of days, and would also eliminate the need for many carpenters on site.

Read my article on How long did it take to build a pillbox? to find out how time-consuming and complex reinforced concrete structures of this type were.

Due to particular requirements of different situations, changes made by builders etc., have resulted in many variations of the basic designs. The type FW3/24 pillboxes along Stop-line Green have a number of variations. Particular to this stop-line, is the addition of a brick built porch protecting the entrance.

All the type FW3/24 pillboxes along this section of the line (with the exception of the Meadow Bridge box) are constructed with prefabricated concrete panels used as shuttering and infilled with reinforced concrete.

The type FW3/24 and FW3/26 used along the Stroudwater Canal and the River Frome are good examples of construction methods used by the use of prefabricated shuttering, thus saving valuable resources and speeding up construction time.

The Stroudwater Pillboxes: A Review of the WWII Pillboxes Along the Stroudwater Canal, Alan Strickland, March 2007

The comments above could also be true for what is now known as the Stent Type 26 pillboxes. In both the Upper Framilode and Stent examples, very specific components have been manufactured to expedite the facilitate the construction of these pillboxes. I believe that considerable evidence remains in the Stroudwater pillboxes to suggest that these defences were also constructed in kit form.

Construction detail


By studying one of the external faces of the pillbox we can break down the methods used in construction. There are four panels used to construct each loopholed side of the pillbox, each panel representing a different mould during the casting process.

Panel 1 has two bolt holes and no loopholes. This type is used at the corners of the pillbox and has angled interior edges making them specific to a particular position. There appears to be a left and right side variant.

Panel 2 has two bolt holes and half a loophole, requiring two of these panels to complete the centre section of a complete side. It may be possible that the same mould may have been used for left and right side loophole panels.

The corner detail of the external panels (panel type 1) shows that they have been cast with a joining angle. This is more complex to achieve than a purely rectangular block and would have required more detailed moulds to achieve.

Some damage to the roof can also give us vital clues on how the pillbox would have been constructed. It is always assumed that the prefabricated panels were used as shuttering for the concrete core, but this image demonstrates that.

A. One of the prefabricated panels. Note the generous join between the panel to its left. These were not poured in situ.

B. The concrete fill is visible. Note the tight join to the front panel. This was poured in situ.

C. The roof slab is different in appearance from both the prefabricated panels and the core. This was poured in situ after the wall core had cured.


Construction of the interior pillbox was achieved in a similar method to the exterior face. Precast panels of varying dimensions were held in place using the through bars visible on the external faces. A comparison of the internal spacing of the bars gives us an indication of why the external bar spacing is not symmetrical. Limitations on spacing are placed on the builders by the internal dimensions. I give details of loophole construction in the following section.

The internal roof of the pillbox also appears to have been accomplished by the use of precast concrete beams. Inspecting the roof shows every other linear ridge to be a result of casting ex-situ, and the other joins are between precast sections, cemented together during assembly.

A full-height Y-profile anti-ricochet wall is present inside the pillbox. It has been constructed using concrete cinder blocks and angled precast concrete blocks. The blocks were likely to have been installed before the roof beams which sit on a seam of cement to adhere them to the wall.


The loopholes are worthy of specific note. It was not uncommon during pillbox construction to utilise precast concrete loopholes, especially in thinner-walled bulletproof pillboxes. In the Framilode pillbox, due to the considerably thicker walls, precast panels were used to shutter the internal surfaces of the loopholes before filling the wall core with concrete.

A precast concrete loophole in a pillbox in Kent. These smaller components were often the only precast or prefabricated parts of a pillbox. The shuttering often being in brick or temporary wood and the remainder of the structure was cast in situ.

Examining the smaller rear-facing loophole we get clues as to how this part was constructed.

A. The thickness of this side wall panel can be seen, as can witness marks from the wooden formwork used during its construction.

B. The gap at the bottom of this panel with the panel below is indicative of two prefabricated panels being placed on top of each other and not being poured in situ.

C. The concrete surface of the loophole has been poured in situ and can be seen to overflow onto the prefabricated panels of the interior wall.

Comparing the detail above to a fully cast in-situ shellproof loophole in a Type 28A in Kent we can see the difference.

Without further detailed investigation, I can’t be certain, but it appears as if the roof has been cast in situ. This is due to the difference in the grade of concrete being used, but also the cohesion with the internal concrete around the loophole.

Parts list

From examining the pillbox construction, it is possible to compile a rudimentary parts list. As no parts appear to have been cast using temporary formwork, I think this pillbox qualifies as a prefabricated variant with only a reinforced concrete fill being poured in situ, along with the roof fill. All of this is of course speculative, but at least well-meaning and well-informed.

  • External wall panel, loopholed (left and right appear the same)
  • External wall panel, left edge
  • External wall panel, right edge
  • External wall panels, rear loophole
  • External wall panels, rear
  • Internal wall panel, loophole centre
  • Internal wall panel, loophole edge (left and right appear the same)
  • Internal wall panels, rear
  • Side panels for interior loophole (left and right may be the same)
  • Internal roof beams
  • Internal loophole lintels
  • Entrance passage side panels
  • Steel tie bar, square washers and hex nuts
  • Steel reinforcement for concrete wall infill
  • Cinder blocks for internal wall
  • Precast anti-ricochet sections
  • Concrete for wall infill and roof

Photographic survey

A selection of my images is on Flickr if you would like to examine them further and in greater detail. You can click the images below for a larger view, or visit the album here.

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    Spotter’s Guide to Pillboxes, Mike Osborne, 2020