It looks to have a "potted" motor on the back of the machine - that is what makes it a 15-91. If it had a little round motor with a belt running from the motor to the handwheel, it would be a 15-90.
There were treadled and hand-crank versions of the same machine, but they all operated in the same way. If you have a manual for the earliest treadle, there will be a few differences (no reverse, maybe different tensioner) but it will thread the same.
You want to check the wiring carefully on a machine of this age - it can have a lot of disintegrated rubber (and a lot of shorts) unless it's already been re-wired since the late 50's. Look for stickiness, cracking or missing rubber insulation all along all of the wiring. If the insulation is sound, you're probably good to go. The wiring from that time period is usually bad, but some have been re-wired with more modern wire that's insulated with more resistant materials than what they had in the 40's/early 50's. :)
I always get the potted motor rewired on these and I replace all the electrical components. It costs about $120 but at least you know you're safe when it's done. :)
The last one I bought was from about the same time as yours and it has the pretty scrollwork on the face plate and inspection plate. Wonderful machine, and I'm sure you'll love fixing her up and sewing with her. :)