I may have figured it out, but I am not positive. Schwinn catalogs list 1020 frames through 1982. The 1983 catalog says 4130 chrome-moly and the 1984-85 catalogs list double-butted 4130 chrome-moly main tubes, chrome-moly stays, hi-tensile forks. The 1986 catalog specifies True Temper T1 double butted chrome-moly and a Tange hi-tensile fork, and the 1987 and 1988 catalogs list the same, but drops mention of the stainless dropout faces first listed in '84.
The 1989 catalog simply says True Temper Full 4130 Chrome-Moly Frame, but I suspect it's the same stuff as before. The fork goes from being a Japanese hi-tensile Tange to being a Taiwan-built Spinner chrome-moly unit, and it is the ONLY Spinner fork listed in the Schwinn spec sheet. The 1990 and 1991 catalogs revert to describing the tubing as True Temper double-butted chrome-moly, with an unnamed chrome-moly fork for '90 and a Schwinn-labeled chrome-moly fork for 1991.
Based on all that, and the consistent 26.6mm seat posts used from the 1986-91 switch to True Temper double-butted tubing listed for every other year, I think it's just badly labeled double-butted tubing.