[Bruce Lancaster]

Yes, master serial number will be there on bell even on the trucks. That first set of numbers should include an 18 or 99 to denote engine type, or H if 6 cylinder.
I don not know much about specific 1941 practices for the prefix, or if any odd stuff entered the prefix on military vehicles, but the actual serial number would be in the 1941 series of either 221 or 239 engines and can be checked agains a list on Van Pelt site. Note that there is not actually a correct number of digits, number of digits depends on actual sequence numbers reached in 1941.