I've often wondered--perhaps Roy's research will help out here--if these vehicles were organized with either a general cargo trailer, or an ammunition trailer to carry ammo beyond the first volley. Or if a 3/4 ton or 1 1/2ton truck was (if only on TOE) assigned.
Even with an abbreviated gun crew, that ought to be a crowded vehicle--so the outside ought to be covered in crew gear. Yet, none of the photos seem to show that. So, I tend to suspect cargo vehicles and/or trailers.