Cro-mo main tubes. 1020 stays and fork.
My '77 was a great riding bike. Got it as a frame + a few OG parts. First built as fixed gear, with 700C wheels (my 1st wheel build. Alex rims, Sansin hubs). 32s fit with plenty of room. The original Dia-Compe 610 centerpulls reached with room to spare. Wald North Road bars, Dia-Compe SS-5 MTB levers. Ended up as a 1x6 with the inner ring paired with a 14-26 Suntour freewheel, using the Shimano fleur-de-lis shifters and a Suntour BL-GT. (Because it was a Suntour, and it was blue, and I had it.)
Mine was blue. Fantastic looking bike. Great ride, though a bit heavy. The worst thing about it was the 7/8" stem... the one the bike came with was too short, and long Schwinn stems aren't a thing. Wish I still had it.
--Shannon