These are the three cab types of Ford gun tractors for the 25 pdr. excluding the Australian types.
"Quad's" or FAT's as tractors for 25 pdr, with the beetle back design where build by Morris, Karrier, Guy, Chevrolet and Ford.
Between the different Ford CMP vehicles (8, 15, 30 and 60 cwt)(Cab 11, 12 and 13) many of the parts are interchangeable. Some parts though exist in "heavy duty" versions for the larger lorries.
There are also interchangeable parts between Chevrolet and Ford but to a lesser degree.
The Gun tractor bodies, on a Cab 11-12 from the nose and rearwards where build by a couple of coach builders and are identical on a Ford and Chevrolet.
I think the same goes for the rear part of a Cab 13, starting from the rear of the original cab.
By all means dive in to the field of commonwealth vehicles they are lovely