Was trying to read-up on this last night in my Hove "Pocket Leica Guide", and they state that the early lenses, w/o the serial stamped in the front lens ring, had the number "scratched into the focusing mount, and is visible upon disassembling the lens"...
So, anyone sending their "unumbered" Leitz lens out for CLA, might want to request that the service techs make note of any numbers scratched into the inside bits, and write them down on the service invoice for future reference...