Since the ring is now a "broken" one having lost the support provided by the metal around the location pin of the FD mount, to make the ring even thinner I attached the ring with the gap to another metal ring (another salvaged bayonet mount from some camera lens I happened to collect!). I added holes to the "holding" metal mount that corresponded to the holes in the ring I was "thinning" so that it would provide the support needed to continue the process in the lathe.