I own Seamaster Automatic Compressor (ref.166.042) with the 565 movement.
This automatic movement is created in 1965 and described by the Omega Museum as the premier calibre / movement, antimagnetic and shock resistant (http://users.tpg.com.au/mondodec/Offcial_Omega_Museum_Calibre_ list.pdf).
What that actually means? It is resistant to how many Gauss? I was not able to find that information.
It was created after the Railmaster antimagnetic (1957) and before the international standard ISO 764 (1973).