The short answer Julian is that the CMP range of artillery tractors were manufactured by both Ford and Chevrolet as:
FAT 25 pounder limber and gun tractors in 11, 12 and 13 cab variants
LAA Bofors tractors in 12 and 13 cab versions
Portee for the 6 pounder anti tank gun
15cwt 13 cab Polsen cannon mounts
A small number of Ford only produced Bofors self propelled tractors which were based on the 60cwt frame.
Ford and Chevrolet CMP's have a low level of commonality other than looking out ward similar (but not identical) commonality exits in some, but not all, body parts, brake shoes and some but not all power train bearings, winches were universally Ford supplied and on later 13 cab variants standard instruments although even here Ford used electrical gauges for temp and oil pressure .
Join MLU Hanno will sort you out they will give you chapter and verse over there
Pete