Thanks Cane. This is somewhat inline with what I read on Indianmotorbikes.com and re-inforces my view for it.
The main one at the moment is in fact the "S" on the other boss area???
See data from Indianmotorbikes.com below:
Engine number suffixes Top
"A letter may be stamped after the engine number (but off to right of boss itself, i.e. on crankcase): these letters are:-
A is for aluminum pistons which were optional over std cast iron pistons in scout 45"in 1928 see note 5
B is for Battery ignition (distributor). (post 1933, see below note) ...BUT
B was also used (briefly) to denote blackout lighting fitted by the factory on mil. cycles during WWII. see note 6
B also designated aluminum piston and/or special clearances for performance and/or sustained speeds on pre '28".
B also designated aluminum piston, chief carb (DLX81?), chief valve springs and chief inlet nipples/manifold used on
special factory 101 Scouts.
BM is for Bonneville Motor.
BY is for Battery (Distributor) ignition on 'Y' motors, i.e.. Y manifolds, trench heads (Ricardo) etc - mostly mid 30's models.
M is for Magneto ignition.
MY is for Magneto ignition on 'Y' motors, - rare, mostly mid thirties models.
6. All Export 741's GDA101 & up were supplied with blackout lighting as standard.
1940 640B's #'s FD01015-FDO1515 and FD02310-FD02636 were also factory fitted with optional kits offered for others.