Hi Chris,
I cannot answer for certain but it is common to find random inspection stamps on various BSA parts
Later gearboxes have the gear ratio code stamped on the mid case STD, RR, RRT2,SC and various combinations.
A late RGSS that I am doing the gearbox on for Paul K has 3 initials? stamped on almost every part of the bike. The stamp style look very BSA????
John