Hope to get some help in this forum.

Plan to buy a "vintage" omega seamaster and found an interesting object at an auction

serial no 22595797
ref no SC165020
Caliber 552

the add reeds ~1965 Omega Seamaster Automatic 18K gold. Recived some pictures and it looks correct. But when checking the serialnumbers and ref numbers with omega website and other sites it doesent fully add up.

the 165.020 is indeed a omega seamaster automatic and should contain a caliber 552 movment acc. to omegawebbsite.
the serialnumber only revel that it was manufactured about 1965. so far so god
But the metal code SC raises my concerns... 18k should be BA, SC is Stainless PVD gray plated, which it not at all looks like. Spoke to the seller who gurantes its 18k gold , lets say it is, could anyone explain this minor confusion?