I got to thinking about single speed engines and whether they were rated as Medium or fully supercharged, so I checked some ratings in Lumsden, British Piston Aero-Engines and their Aircraft.
It could be the case that several variants of a particular engine would have the same rating, even though they had different mark numbers.
The mark numbers would change for different reduction gear ratios, maybe different accessories, or a number of other reasons.
The Merlin 21 used in the Mosquito used reverse flow cooling, but it had the same rating as the Merlin XX - RM.3SM
R = Rolls-Royce
M = Merlin
3 = engine rating
S = fully supercharged
M = medium supercharged
This was a 2 speed engine.
For early Merlins, the Merlin III was rated RM.1S and the Merlin VIII RM.3M.
The Merlin X, a 2 speed engine, was rated RM.1SM and the Merlin XII (single speed) RM.3S.
The Merlin 45 was rated RM.5S and the Merlin 45 RM.5M.