That is a Schwinn "Girder" design "S" stem (p/n 55 614), which had a 3.5" extension, a 7" vertical stem length, a 0.833" (21.15mm) quill diameter and which was used on most Schwinn lightweight bikes with 24" and larger frames from 1973 through 1976. Bikes using that stem during those years include the Sports Tourer, Super Sport, Sprint, and Continental. Note that only 24" and larger frame bikes received that stem, the 22" and smaller frame models used a rounded design "S" stem (p/n 55 615) with a shorter 2.75" extension and 6" vertical length. All of the girder (and most of the rounded) design "S" stems were stamped "Made in Britain" at the base of the quill and were made for Schwinn by GB (Gerry Burgess).