Guess it's not super critical, but measured some I have. The total length is 4-3/16". The length I think you've indicated on your picture (from the square corners at the base to the end) is 3-11/16". Threaded length is 1".
Main reason for posting is to see if anybody knows if these are sold anywhere? Guessing they're the same as on some later trucks right up into the 40's, but haven't had any luck finding anything.