Hi All,
OK, the truck-mounted gun is actually a British 4"50 cal. Mk 7 which probably came out of storage after being mounted on an early dreadnought: Here's a picture:
Key things here are the squarish carriage, and the recuperator mounted to the top of the barrel. The US 4"50 cal used on Clemson class DD's had the recuperator mounted below the gun.
US Clemson class DD's that were lend-leased to Britain didn't begin refitting in the UK until 1941, and in most cases, since these were navalized, they were then remounted on merchant ships since they were built to resist salt corrosion. Placing them on land, while possible, would have been frowned upon by the RN due to the need of modern weapons on merchant ship.
Jon