To convert the Airfix Stalwart Mk.2. 
The Stalwart was a front-supply High-Mobility Load Carrier, an off-road truck that could follow and reach tanks and artillery units – capable of negotiating knife edges and the like, unlike normal trucks. Although deliveries started from mid-1963, the Mk.1 was officially accepted for British Army service in March 1965, and operated alongside the Mk.2 for several years. 125 were delivered, all with EK vehicle registaration numbers (VRNs). The swimming role of the Stalwart was only in case a bridge was not available. Swimming was not the primary role of the vehicle. 
In 1964, a small number were shipped to Yemen for hot-weather trials, taking part in operations against the National Liberation Front in the Radfan. Most later saw service with BAOR in Germany, with others used by the Royal Army Service Corps (RASC). Tyres were changed from Trakgrips to Bargrips whilst in Aden, due to the stony roads shredding the Trakgrip tyres. Later, all Mk.2s had Bargrips from production. 
Stalwart Mk.1 is easily distinguishable from Mk.2 by much smaller, rectangular windows all around. Many other differences are noted below. The Mk.1 had a different internal cab layout to Mk.2, with the vehicles batteries located to the right of the driver. Following injuries from battery acid splash after mine detonations in Aden, a modification saw a metal partition installed between driver and battery box.
Like Stalwart Mk.2, from about 1982, the amphibious role was abandoned due to modern bridging technology. Water jets and trim vanes were removed from many Mk.1s, but not all. Mk.1s were NOT fitted to carry radios due to carrying fuel and ammunition, but did have two round aerial blanking plates on the roof. With the introduction of Mk.2, some Mk.1s were also fitted for radio (FFR), but not usually equipped. Some Mk.1s had Mk.2 modifications before they left service, such as double silencer from 1972, the addition of mudflaps, rear number plate holder, and relocating the airline connectors onto the hull rear plate from the hull sides.

Differences Mk.1 to Mk.2 Stalwart
• Cab top differs in a lot of details including slats (not grilles) over the air intake. • Horn on right cab side. Fuel filler on roof.• Rectangular windows. Outer front windows could be dropped down like the rear side windows. • Different step arrangement on cab sides. • Cab interior different in many ways including split instrument panel. • Only one wiper on the central windscreen. • Different wooden water deflector (with Alvis badge) and simpler (but difficult to use) support system. • Cab top air deflector more angular. • No eyes on the nose or rear hull. • No tow point on the nose. • Strengthening slat lower down on the cargo sideboards – no vertical bars. • Single silencer. • Different water jet housing. • No mud flaps. • Tailgate hinge/taillight arrangement different. • Rear hull plate was 1-piece. Mk.2 was 2-piece. • No rear number plate mount. • Air connectors on rear hull sides. • Tilt support (mounted inside load space) was height adjustable for various load sizes.