My '61 S/R is stamped DA10R **** but no HC or HHC. I believe it's an abbreviation for High Compression and do remember my '63 reg. S/R back in the day having those marks stamped on the c/case.
Also, whilst having the cyl. head off recently for repairs i noted the pistons were flat top, not raised or domed. Is this likely to be standard or is it possible that a previous owner has fitted lower compression items.
Any info gratefully received.
The SR cases were usually stamped HC for high comp or HHC for high comp and high lift cam.
Flat top pistons would normally be low compression, probably a number on the top if BSA or old Hepolite which would identify them.