Okay kwendt, I know I'm a nut, lol. I found this site that has all the info you are looking for (and so was I). From this page you can access a lot of the machine feet, repair parts, and instruction manual for the 15-91 (and many others). As soon as I opened this site...Bam! There was my grandmothers machine! I can not tell you how many machines I have looked at that are marked as 15-91's on Ebay that are apparently NOT! No wonder it has been such a confusing chase. Okay so here is the link:
http://shop.sew-classic.com/Singer-1...-15-88_c27.htm
Have fun, sorry for adding to the confusion!