I think this subject deserves a thread of its own.
Rebuild serials were sometimes issued to a rebuilt machine but judging by the number of M20s in the post war key cards considerably over 3 years of age still listed with their original factory "C" Serial numbers it wasn't always the case.
This booklet offers some clues, the reference could date from 1942 or 1944 depending on whether its in the original A.C.I (Army Council Instruction) or the amendment?
I think it says that to be a Class I vehicle it needs to be rebuilt every 3 years, otherwise it becomes Class II? but that doesn't stop it's use??
I don' think that I've seen a plate referring to a Class one Rebuild dating from earlier than late 1944...nor have I see evidence of the rebuild series in use much before that, but we know from magazine articles that the practice was being carried out...There are also too many contract numbers that don't match frame numbers in the Key Cards which suggest to me that old WD Serial numbers were being re-used almost at random at one time ?