Andy,
I've written about these mounts on here before so do a search as I have ammassed a lot of information and done much research, but in brief the one on the back of the truck (CMP, Bedford MW and apparently WOT2) was called a universal mount and is very different to the airborne mount.
There was also a universal type mount for the Hispano 20mm gun and a number of trialled jeep towable single mounts that never took off. There was also a 2 wheeled airborne lighweight Bofors 40mm mounting and one survives, very nice mount.
Carleton