Hello Ian.If you take the timing panel off the same numbers should be stamped on the outside of the R/H side crankcase ,also on the inside of the L/H crankcase up the top.Probably to keep the 3 main components together during overhaul.On the BSA WM20 they have matching numbers on the front for the same reason.Sam.
I've not seen one with a number above before but all Norton engines of this period have an alpha / three numeric code for matching.
The cam bushes and main bearing housings were line bored so it was necessary to ensure that the components which had been machined together were matched on assembly. The code seems to have gone from A001 to Z999 and then been re-used.
Up until the middle of 1940, it was also common to see a build date stamped inside timing cover and crankcases.
