Wow nothing like home work guys, no pain no gain as they say here are the answers to the various questions. I will post the pictures with a couple of replies in a moment
Stub axle length, this is an approximation with out removing 3 1/4, diameter 7/8
Left swing arm slot 0.681
Sprocket teeth 46
Back plate hole 0.878
Bearing in hub OD 2 inch ID 7/8
Total hub width from outside edge of spline to outer edge of right side hub 4 1/8
Stepped spacer hub bearings sit on is 3.450 length
You will find things a lot easier if you keep to fraction of an inch and forget decimal measurements.
0.681 ~ 11/16" ( 0.6875" )
0.878 ~ 7/8" ( 0.875 ")
BSA used fractions and all parts are measured in fractions.
Now I am no metrologist but when it comes to these type of measurements it is micrometers in multiple places then averaging because we are talking about parts that are 50 years old and subjected to all sorts of bodgery.
Get some fuse wire or hair wire and measure the axle slots diagonally .
Good chance is you will find the two sides are not parallel.
Next of course doubt every part. No reason to suspect any part on the bike is what was there when it left the factory.
In the USA where they were toys there is a good chance the owners made the effort to get the right parts for their bike.
Down here or in the UK where the bike was daily transport then whatever wheel the owner could get from a wrecker ( because BSA riders were too cheap to buy new parts ) that would go in the hole & work or be made to work was better than the No 52 bus + 3 minute walk to the No 106 and the chance of making the boss angry by being 3 minutes late..
Then you get collision damage where the swing arm is bent back to put the wheel into alignment. I had a bike done & it was fantastic , never tracked that good before, but when it came time to do the swing arm bearings the shaft would not come out. Carefull measurements confirmed that the pivot was in fact bent & it has to be cut like devon to get it out.
Finally to help you & us identify your wheel & swing arm & wheel, lay the entire axle out parts book style.
Usually if done like that you can compare it to the diagrams in the parts book.