This is not true about about the SIII FFR at least.
I have seen both 6006 (Truck, Utility, 3/4 Ton, GS, Fitted For Radio) without the suffix letter and my FFR is 6006B (Truck, Utility, 3/4 Ton, GS, with winch, FFR, 24 volt system) and the suffix letter "B" stamped on the nomenclature plate although you have to look to find it.
In the "Census No." field, you see the 6006 aligned to the LH margin of the field, but the "B" suffix letter is aligned to the RH margin of the field.