Singer made model 15 for many years, I have a couple of them. The second numbers (after the 15) like the 91 refer to updates that paticular one has. As the technology improved so did the Singers and the model #s changed. They may have motors, different bobbin winders, different stitch lenght adjustments. The 15-91 is a later model and very nice.
Can you tell I have way too many old Singers?
Good luck with it and do ask for many closeup pictures, front-Back_Bobbin area etc. Mary