I've got a large medallion caliber 501, but the stainless variety: http://omegaforums.net/threads/1950s-omega-seamaster-gold-bezel-cal-501.6908/ Don't know why they didn't put a gold medallion on a two tone watch.
Seen the two tones both ways. The only variation I don't have is solid gold. Most unusual variation that I have is pink gold capped with pink gold medallion.
Is your two tone a gold bezel model or gold bezel with gold capped lugs/top of case? My gold bezel only, has the stainless back.