I have a patek automatic sub second hands 37j movement from the sixties with a factory display back. I'm the second owner and bought it in 1973 from the original owner's son. The factory confirms the watch is authentic. I think it's ref 3561. My question is whether this is the first series Patek made with a display back? If so, how many pieces were made. Any other reliable information on the history7 of this ref would be appreciated
