That's a Gargette (RGF) BB shell. It has been confirmed by a number of people on the C&V forum. Another guy took pics of his Proteus and while it's a bit newer it's nearly identical except the Pd cut out under the bottom bracket shell. So.......Pd for Proteus designs, identical lugs to many of the Proteus frames I've found and several pics had the same seat cluster design which is also outlined in their framebuilding book which I have.
So ya! I'm almost 100% sure it's a Proteus. If only I could find a pic with a bottom bracket cut out like mine or maybe a first hand account by one of the many frame builders that worked there in the early to mid 70's.