Thanks for the comprehensive info! By "plugs" I think that I have figured out that you meant the end plugs on each seatstay.

I've come across those plugs before, on an Italian mystery rust-bucket frame, oddly enough the complete parts ensemble of which had been carefully boxed and stored indoors, including a new-looking Brooks Pro saddle. The only indication that I found of why it was separated from it's parts was a rear hubshell with flanges well out of parallel, as if only the rear wheel had perhaps been run over by a car. I was told at the time (15 years ago) by helpful forums people that it was likely of one of the brands (Ideor, Atala, etc.) from some particular region of Italy. I still was still quite happy to have been gifted all of the parts (including a few Magistroni bits and a Stronglight 57 crankset), and a local friend used the rusted frame as yard art.

Again, thanks for the additional info. I'm heading out the door now to take it for a test-ride, after having restored free movement to the brake cables and adjusted a new WTB Speed saddle. I don't think that this bike has been ridden in about 20 years.