Everybody's guessin'
BERTIN? There's NOTHING about that frame that says Bertin!
The "Milremo" dropouts were supposedly made by Mavic but they were more likely made by one of Italian companies like Technociclo that made most of the forged steel dropouts back then. Andre Bertin as well as Ron Kitching sold those Milremo/Mavic dropouts. I bought a couple of sets back ~1977 but never used them because of 3mm or 4mm threads for fender/rack mounts. It looks like the holes were drilled out to 5mm in this frame.
The BB shows silver brazing. My guess is that it was a small builder or amateur in the US. The Brits were too cheap to use silver and not many builders used those dropouts on either side of the pond.
Ron Kitching sold some Andre Bertin frames from the late 50's into the 70's. One problem, Bertin frames had French metric threads. Supposedly a few were made for the UK market with British threads but I've never seen any evidence confirming that.
1957 Kitching Bertin ad:
1960 Kitching catalog:
So to guess what the frame is??? Based on those dropouts and the silver brazing I suspect that it's a US built frame from the late 70's. Some braze-ons could have been added before a respray???
Good luck in your search but lack of pedigree shouldn't affect your appreciation of the frame.
verktyg retro grouch