Being the owner of 9 SSs and one 1972 Sports Tourer the cast forged drop out disappeared in the late 1971/early 1972 models and Schwinn went to standard for them Continental rear drop outs on SSs. The change was about the time the Sports Tourer came out which then because it was a better bike got the cast drop outs. There is a lot of differences between my earliest 1966 bike and my last 1973 model and they all have to do with making the bike cheaper. Roger