Bkay, for a 15 class machine the numbers after the dash are designations that mean that model has some sort of change. For example, a 15-88 is a natural born treadle. Your 15-91 is an electric 15 with a potted motor. There are several 15 class machines: 15-30, 15-75, 15-86, 15-87, 15-88, 15-89, 15-90, 15-91, 15-96, 15-97, 15-125. I think I remembered them all but there may be more. Each had something different about it while it was being produced.
Cari