Steve, I reckon GB's diagram nails it as part of a possible answer. The design appears to allow the rollers to stay in the case, the crank pokes through and the ring is the inner part of the outer axial support for the rollers. If we are right there should be a nice even witness mark around the periphery of the ring where the rollers have been. As a design alternative it is a clever trick, avoiding damage to the cage and rollers as the inner race is taken on and off to shim up and set the minimum running clearance to the face of the timing bush, as once assembled there will be no end float apart from the axial clearance in the bearing as the rollers are supported "top and bottom and both sides" The only weakness is the ever critical tightness of the big nut pushing the inner cush drive sleeve and the ring hard up against the bearing inner. If you still have the original main bearing to hand, see if the ring fits.
The only other part that looks remotely like that ring is the rear sprocket hub collar, 67 6078 which fits in the centre of the brake drum/sprocket, around the rear spindle. No need to remove this on a regular basis, most are "rusted" in place into the drum. It is just a plain metal ring, no posh numbers. If yours is in place, then this mystery bit is spurious to your build. Otherwise this part may have been fitted there as a codge.
Admiring your hard work and attention to detail. Josef is in for a treat.
Swarfy.
