Michael, the earlier series all had serial numbers starting with 7 and the second series with red markings all started with 8 (as far as I know). I had one of the earlier ones but surprisingly won a later one on eBay with a very low bid, so I ended up with two. I did some comparison tests and while they are very similar, the later one seemed to have slightly more field curvature, judging by the shape of OOF highlights near the edges.
By the way, for those without the jam nut, a possible solution using the BR15 or wanting to mount on an M42 helicoid is to get a M39 to M42 adapting ring on eBay. It is about 3mm thick, which might solve the rear clearance problem. Of course you can use more than one. Not beautiful, but it is solid and it works.