On pre-monbloc carburettors the suffix "M" denotes
"During the year 1950 all type 14/ and 64/ series or complete type 1 and 2 Float chambers were modified by increasing the diameter of the feed bore in the arm of the body. This necessitated the increase in size of the plug screw and washer, the new parts numbers of these being:
PLUG SCREW 64/353 and PLUG SCREW WASHER 23/110
All modified float chambers bear the suffix letter "M" EG a No 1A float chamber modified would become part No. 1A/M. "
Source AMAL carburettor settings and spare parts list 1940 to 1954 (inclusive).
Speedo Cable dimensions
The nominal length of a flexible drive is measured between 'A' and 'B'.
Bowden cable (Choke, clutch, brake etc) dimensions
Are measured by denoting the OVERALL length inside the nipples.