R Cubed,
Vehicles made in the USA during WW2 and right up to around the 1980's, would certainly be using inch sizes, their threads being predominantly American National Course and American National Fine. Spanner sizes will be AF, which means "Across Flats", so to determine a size of spanner / socket required, measure from flat to flat, with an Imperial vernier or caliper to save having to convert. Do not fall into the trap of what some people do, when measuring a bolt head, for instance, 9/16"AF spanner size, they then think it is a 9/16" bolt, the actual shank diameter would be 3/8".
Forget the 105mm gun, armaments are a rule to themselves, but I have no doubt that the gun carriage would use American inch threads on it.
Richard