Royal enfield frame dating
In the few photos I have of these stampings, the prefix is on the right hand half and the number is on the left.Please note that this is right and left as seated on the bike, not as viewed from the front.The prefix letters also change over subsequent years, becoming SM with furher letters for both 500 and 700cc machines from 1957 on, eventually changing to other prefix letters altogether from 1959.I am not going to make any attempt to explain the significance of these changes.Up until sometime in 1956, a "true" engine number was stamped across the front of the crankcases.These numbers have a W or W5 prefix for 500cc and (usually) a W7 or SM prefix for 700cc.
These units were then sent to Redditch where they were fitted to the cycle parts to build a complete motorcycle.
Later engines have all of the gearbox number on the right hand half only, and I suspect this coincides with the engine number being transferred to the side. as I do not have enough photographic evidence to sure of this claim.
If the number on your gearbox does not match that on the front of the engine, then either the gearbox or right hand crankcase half have been changed.
This custom and practice was obviously entrenched, as it has become apparent that not all of the non-matching engine numbers from 1956 onwards have been recorded either.
Thanks to the internet and helpful owners, I have managed to harvest a few of these missing engine numbers.