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The Bison extemporised armoured fighting vehicle, frequently known as the mobile pillbox, was produced in Britain during the invasion crisis of 1940-1941.
Invented by CB Mathews and using the original pillbox concept from the First World War, the mobile pillbox design was based on several different lorry chassis and featured a fighting compartment protected by a layer of concrete.
Trademarked by Concrete Limited, Bison became the generic name for this type of vehicle.
Concrete Limited obtained chassis where they could, many of them were old and dated from the First World War. With a variety of chassis to work on, Bisons inevitably varied in detail, but were made in three distinct types:
Type 1 was the lightest. It had a fully armoured cab and a small armoured fighting compartment roofed with canvas.
Type 2 had an armoured cab roofed with canvas and a separate fully enclosed fighting compartment resembling a small pillbox on the back.
Type 3 was the largest and heaviest. It had a contiguous cabin and fighting compartment completely enclosed in concrete armour.
Steel was a valuable wartime resource, so concrete was favoured for armouring these pillboxes. Any available lorry chassis was used, although the weight of the concrete meant that only the heaviest of chassis were suitable.
At Concrete Limited’s Stourton Works, the lorries’ original metal bodywork was removed, and wooden shuttering formers constructed. Within the formers, three sheets of expanded metal were placed as reinforcement and then fast setting, high strength concrete made with high alumina cement was poured in. Characteristic ridges at the gaps between the boards were a noticeable feature.
When a roof was provided, this was constructed from pre-cast concrete. The walls were about six inches thick and were found to stand up well to Bren gun and armour-piercing bullets.
It is not clear how many Bisons were produced; estimates vary between two and three hundred. Owing to the resilience of concrete, traces of Bisons can still be found today.