Posted this three years ago and got no replies...so it is possible that these R stampings are not all that common. Well I think I may have found the answer. The engine is from my '39 VH and I think the 'R' stamping may relate to the engine being fitted with Roller bearings. Apparently pre war Matchless sometimes also stamped an R to identify Roller bearings being fitted.
Vincent...well that seems to confirm it was a factory stamping. I've had a look through the photos of my restoration and there is an R stamped on my timing side crankcase in the same positions as yours. If your crankcases are from a machine that would have originally had rollers both sides i.e a VH then I think it is safe to assume that the R denotes Roller bearing OR as you say Racing .