Singer 15s have the tension on the left side on the face plate. I can't think of any other domestic Singer that has it there. The Singer 15-91 is a 15 with a potted (built in) motor. "Newer" model 15s with reverse have the circular plate around the stitch adjust lever. Older ones with no reverse have a knob that you could move up and down but no round plate.
It took me a while to spot the differences and I've made more than one mistake trying to ID machines. I'm sure I'll make plenty more.
Rodney