I'm glad I finally clicked on the link. That frame is almost identical to the first frame I ever built. And when i say that, I mean the lugs and all of the fittings. I didn't really remember what the fork was like, I seem to have gotten rid of it, but seeing this reminded me. I wish I had pictures to post for you. I really don't remember where I got the parts, but I'm pretty sure it was from Proteus. I think they may have had kits, and this may have been one of the kits. I don't think I had all the braze-ons that this frame has. I just had my hands on the frame yesterday. It has never been assembled and it is almost 35 years old.

The frame may well be a Proteus, but it may well be a homebuilt. I'd worry a little about the fork, that isn't an easy joint to braze properly. Most of the Proteus built frames that I saw are very similar. One aspect is the seat cluster. It was like a fastback, but the stays went alongside the seat lug. I don't know that they ever made a frame like this.