Tim, what Rod and Mark said is correct. All WLAs for 1942, 43, 44 and 45 were originally stamped 42WLA. Belly numbers for 42WLAs could begin with 41, 42, 43, 44 or 45.
According to Bruce Palmer’s 1932–52 military book the Model 42WLA was produced almost continuously from September 1941 to August 1945.
If the WLA motor you found in the car is all factory-stamped and the belly numbers begin with 45 then you may find the serial number to be higher than 60000. But please note my opinion is based on examples I have run across so far and it isn’t a definite because sometimes authentic numbers can be far removed from what I’d expect to see under normal circumstances. However, something I would certainly expect to see in the serial number is a certain type of open-top 4 as opposed to the certain closed-top style used by H-D early on. (For 42WLAs the 4 in the serial number seems to have changed from closed-top to open-top somewhere after 43000 and prior to 48000.)
In his military book Palmer mentions two possibilities regarding why WLAs made for 1942–45 were all stamped 42WLA but I do not know if either of those reasons is correct.
And I read a third explanation elsewhere by someone else but I do not know if that one is correct either.