Dear DrStrong, you cannot imagine how grateful I am for your help
I was looking after the reference since the acquisition of the watch a few weeks ago. The previous owner was an university professor but was not a big watch expert and couldn't help me, I asked other collectors but mainly Omega lovers which had no idea, I googled the whole internet but no way and as you can imagine this thing made me mad. So clever I am, I looked at 12 o clock and there was the serial number but I did not even think to look at 6....
By the way, it is a ref. 7934 like you said. 
Thank you again. May you have a healthy and successful 2016.
Cheers from Switzerland,
Pietro