I have BSA WM20( frame no WM20 115248 (1945) and engine no WM20 103748 (1944). Most of parts look fine, but there is one problem. Some of them have strange numbers for example: chaincase front 66-7503 (parts list no 66-7502), chaincase rear 66-7602 (parts list no 66-7601), rear stand 66-4773 (part list no 66-4772), gearbox outer cover 66-3103 (parts list no 66-3102), ball head clip yoke 66-5141 (parts list no 66-5138). What does it mean??
I didn't take a look at the parts lists yet but I guess that some of these parts numbers could be for post-war M20 parts. As these bike were used for many years after the war lots of spare parts are made for post war models and can be slighly different. Often the headlight was replaced for a MU42 and the later model gearbox etc etc.
Numbers that are cast. stamped or forged into a part are not the actual part number.
They are the number for the raw part before it was machined , welded , painted etc.
Usually they are from 1 to 5 units away from the finished item numbers.
A lot of times the same raw part could end up as two ( or more ) dfferent parts.
Kick start quadrants are a good example of this,
A clearer example is pistons.
Std & + 20 are both machined from the same blank
but for + 40 & + 60 they needed to use a bigger casting.
