Well I found something:

about 1/3 of the way down the page is:

which clearly show the grooves for that race in both the back plate (theres even a red arrow pointing to it, middle image). These posts look to be about Gold Stars but Magnetoma makes no mention of these bearings which is odd considering the detail he goes into.
Is that what I have? A GS clutch?

Murray

email (option): murray [at] littlewoodmansion [dott] com

This shows that there is no end to the learning curve...In 50 or more years I have thrown a lot of six spring clutches over various hedges, removed from both single and twin cylinder BSAs and this is the first time I've seen the 'extra' race...
I'll certainly take a look at some of my BSA documentation to see whether I can find any information on this...A Gold Star connection is plausible but earlier versions (up to the adoption of the Triumph clutch on the swinging arm models) weren't quite as 'specialised' as the later big fin types and generally conformed more to the layout and specs. of the more mundane models, apart from the alloy top ends...
However, this had to come from somewhere!...Another question I must know the answer to!!!....Ian

email (option): ian@wright52.plus.com

Looks similar too a pre war BSA Blue star clutch.

email (option): fuzzyempire@hotmail.com

I cleaned up the back plate hoping to find perhaps a part number:

No such luck.

I don't understand what this bearing would do other than prevent the rear friction part of the chain wheel contacting the back plate. Assembled on the bench with a few balls and pressing the two together the chain wheel slides past the backplate with a bit of drag. Take the balls out and it slides with lots of drag. Isn't that what one wants from a clutch? Its also apparent that if the lift on the pressure plate is too much then the balls can escape the races - the grooves they run in are not that deep.

Murray

email (option): murray [at] littlewoodmansion [dott] com

The 6 spring pressed metal clutch hasn't really stood the test of time. I don't remember having any trouble with them back in the 70s but like me they're pretty worn out now. So I replaced mine with a Triumph one from Kidderminster Motorcycles.