For me, looking at lots of photos, reading up, asking questions. I can say that the 15-75 (belt drive) and 15-125 (gear?) do not have the old style silhouette. They have a more modern, industrial look.
As I understand, many of any Singer old style machines, the model number variations are due to small design changes.
(Often times different manuals are found in cabinet drawers. The basic information on oiling is still very helpful, no matter the model.)
You already know the three major versions, treadle -88?, Belt drive -90, and potted motor -91. Here is a link to a chart on needle bar. I can't do a screen shot so I hope this link is ok.
http://www.needlebar.org/main/15chart/index.html