[David_Hayward (RIP)]

My book suggests that the 9XX series was by 1940 the heaviest (weight) line. I can imagine that the military trucks were placed in that series as they were the biggest designs, and then the next two numbers represented allocate Drawing Office model numbers. As you know GM of Canada assigned the "8" prefix before the number of wheels, and then body style because it was not used..."2" was Pontiac for instance, and "9" GMC. In fact "8" was not used until 1962ish when GM of Canada produced their Acadian series based on Chevrolet running gear with Pontiac style trim for Pontiac dealers.

To recap:
842X = CMP 4 x 2
844X = CMP 4 x 4
86[60]= CMP 6 x 6

I see no reason why Diamond T did not allocate specific military model series likewise.