Schwinn as well as several others (Ross, Windsor and Nishiki come to mind) used a .833" quill stem that we metric-minded translate to 21.1mm. It was one of the "standards" for them for a while, also showed up on department store bikes and some other bike-boom Japanese brands, but got left in the past when JP adopted the 22.2 standard. You'll find better results searching for those stems using the "inch" ( .833") designation as well as the metric translation.