Good question! The class 15 machines came in my styles, Ismacs has excellent information on all Singer machines.
http://www.ismacs.net/singer_sewing_...sses-1-99.html
Scroll down this page to see a description of every 15 ever sold. If you check out the gallery of machines on this website you'll most likely find a photo of your machine that will help identify which one it is.
Good luck! Have fun!! I have a 15-91.