In researching the various keys used on Alpines and Tigers, I found there were as many exceptions as consistencies. It seems that each car got one key for the ignition and doors, and another for the trunk and cubby. Most Alpines used RM or RL, but this was difficult to document. Early cars may have used FP or SR, and ST was mixed in. My 1962 HLM has RM. There was consistent use of the FS key and lock for trunk/cubby. The letters delineated the type of lock the key would fit in, mainly by the fluting size and spacing (the longitudinal groove). Sean is right that RM and RL are essentially the same and interchange. There is a slight difference in the shape of the head. The ST keys and locks were mirror images of RM/RL, in that the fluting was on the opposite side, and therefore ST and RM/RL will not interchange.
The FS keys had fluting on both sides, but RM/RL/ST only had it on one side. Therefore they will not fit in an FS lock, but FS keys will fit the RM/RL/ST locks. I've wondered if it would be possible to get a single FS key set up to work all 5 locks on the cars.
Tigers mainly used RM keys, with none using RL. There were 811 ST keys/locks in Tigers, mainly in later Mk1A cars. Mkll are almost all RM, with only 3 ST. The Jensen ledgers are really helpful here, but of course there is no equivalent ledger for Alpines. All the Tigers used the FS "Union" key for trunk/cubby.
The RM/RL keys usually had Rootes on one side with the letters/numbers, and L&F and an arc of Made in England on the obverse. ST keys had Strebor instead of L&F, and the ST... under Strebor. Later keys had the Chrysler pentastar. Replacement keys were either blank or had WBH on them. David