I know nothing about M3Ws but I have seen that oval slot in the past on cars. It looks like an inspection slot to check wear on brake shoes without dismantling the drum.

Last edited by Deejay; 30/11/19 03:40 PM. Reason: Spelling error
Doug
2011 Plus 4 in Rich Maroon

1972 750 “ComDom” sprinter
1958 Triton 650
1992 Triumph Trophy 900