Mine is 1125 on a Zenit 6 body 6500175 (would it make the camera #175 or they reset the numbers every year?). So if it is camera #175, there must've been 950 Rubins made before the release of camera, maybe all those were M39? Or am I making thing too simple for reality?
Zenit serials do begin from 00001 every year. Lowest that I have is my AM2 92000002. And lens serials rarely match camera serials in any way, even the year can be different. So in my opinion many cameras/lenses were at stock for a long time before packing.
Rubin for Zenit 6 and M39 probably have separated head count. IMHO may existed same s/n for both mounts. M39 should be extremely rare. I sow few Zenit 4 lenses (Vega) adapted for M39. Probably Rubin lenses was adapted same way too. But lens on the picture IMHO is genuer; it has different profile then Rubin on Zenit 6.