Need help with ID of frame manufacturer
Bought a bike at an estate sale about 12 years ago at a very reasonable price.
Bike appeared to not have been ridden in years, frame appeared to have been repainted.
Bike had no decals, but nice lug work with cutouts.
The only identifying mark is a raised letter "M" (3/4" x1/2") braised on the down tube, 2" below the shifters.
The rear droputs appear to be stamped with "Shimano ST". There is a "268" stamped under the bottom bracket. The bike had all Campy components except for Gran Compe brakes.
From components, I would guess it is late 60's to early '70s vintage. Rear Nuovo Record deraillleur has no patent date. I enjoy riding the bike and would love to know if the raised "M" was used by a bike/frame manufacturer and could give a clue to the maker.
Thanks for any help that can be provided.