kwendt,
Your Singer is a Class 15, not a 15-91. It was made in Elizabethport, NJ on March 3, 1942 along with 24,999 others made the same day. I have the same model in a 1946 version made at the same factory. My grandmother bought it new in 1946 from a dealer near Wesley, IA. It's the machine I learned to sew on when I was eight years old. It still works and is in the original cabinet with the knee pedal. Mine needs to be re-wired. I have always loved sewing on it more than any other machines that I have owned. It sews through anything!! The Class 15 seems to be more rare to find these days as I have been looking for one (for parts if needed someday) for a long time. The cleaner and more pristine the condition, of course, the more valuable it is. These old Singers often go for more than $600 at auction. I think you may have found a winner...I hope you can get a hold of it. Good Luck!
Edit: I may be incorrect in stating it's a Model Class 15, according to All Brands.com they are stating that the Class 15 is the treadle machine and the 15-91 was the motorized version. It's confusing because I have never seen a 15-91 that has the same markings on the machine etc as the one that I have and some of the parts on mine that are silver on mine are black on yours. I'll be watching to see what others may have to say. I'm sorry I've misled you.