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 
email (option): ian@wright52.plus.com
Hi Ian, thanks a lot for that. The number is actually on a bike that Bonhams sold last April as a circa 1939 M20. No documents. Fetched £2,760.
Here's a link to a Sump page with the bike. I'll make some adjustments.
http://sumpmagazine.com/bsa-m20-500cc-sidevalve.htm
Please remember to keep a battery box for me. Mines a 1945 model, but I'll take whatever's available.
email (option): dannydefazio@sumpmagazine.com
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
email (option): ian@wright52.plus.com
I'm just being picky for the fun of it...It would make a nice bike to run around on...Ian
email (option): ian@wright52.plus.com
"Picky for the fun of it" has a name, Ian. We call it "Rivet-Counting"....
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
email (option): ian@wright52.plus.com