Hi Danny...ZM20 6001 was the first M20 engine of the 1951 production year..ZM20 10001 was the first ZM20 engine of the 1952 production year.
The number ZM20 7475 is 1474 into that sequence (6001-10001) so a guess based on the number produced that year would put it just over a quarter of the way into the 1951 season.
BSA numbers ran from August to August so I'd say about Nov/Dec 1950...
In that year it could have been rigid or plunger and would have had telescopic forks...Ian
It'll be a 44 dated box for you then Danny...That M20 has a postwar petrol tank and primary chaincase as well..The gearbox plates that take a centre stand (and the stand) come from a 49-54 plunger model. The winged timing cover is also 48 or later and thus never fitted to girder fork models, handlebar levers are also postwar..and it needs a new kickstart spring.. ...Ian
Interesting...My 'guess' was a few months out....
Rigid frames for that year started at 10001. However, the plunger frames commenced that year at ZM20S 101...So a number of ZM20S 177 would have made it a pretty early plunger model..
4000 rigid models were produced in that year but only 200 of the new plunger model according to the information I have...Ian
I had just put 177 in my post as frame number as this ledger doesn't say if it's rigid or plunger. It does mention that it's equipped with S.F. but I have no idea what that could be. (spring frame???)
These ledgers are put on micro film a long time ago and it has been done by people with not interest in old bikes at all. So they didn't look any further then necessaries I am afraid, I guess that some of us could have done a far better job with these ledgers. An other problem is that from models we only have the engine ledger and from other the frame ledger. As BSA made a real mess with combining engine and frame numbers it's nearly impossible to find the one in the other ledger.
But all in all I am very (absolutely) pleased having these ledgers of course.