Originally Posted by
verktyg
T-Mar, not necessarily the alloy but the wall thickness of the seat tube which can be a good start for guesstimating...Beyond this info, it's just WAG - wild a** guessing
Tubing wall thickness is directly related to the strength of the steel which is a property of the alloy. Provided the frame was not built for a specialized function, one can deduce the alloy based on the state of the art metallurgy for the industry and era. Of course, there will be some overlapping "gray" areas, where there is more than one possibility, but it's far from WAG.