if the seatpion size is 27.2, youve got a good un. Probably reynolds 531/753 or columbus SL.
If the seatpin is around 26.4 then youve probably got cheapo columbus (aelle or similar), plain gauge reynolds or falk. The first 2 are still quite good, just a bit heavier. Falc was quite nasty - altho serviceable and the basis for a good commuter, I think it was seamed tubing. Look in the BB and see if you can see seams on the downtube or seat tube.
my mate bought a 753 harry quinn in the late 70's and has only recently chucked it cos it broke. Raced as a first cat on it throughout the 80's. Rated it far higher than his carbon giant and was sorry to see it go.