That's looks more like an Omega cal.625 to me.
http://www.ranfft.de/cgi-bin/bidfun-...uswk&Omega_620
http://www.ranfft.de/cgi-bin/bidfun-...uswk&Omega_625
Genuine cal.625 but definitely not a cal.620. Both from the same family so interchangeable but the 625 runs a little faster.