You paid a good price for it and you will love it when you start using it regularly. If it has the potted motor (looks like an old style pot next to the handwheel) it is a 15-91. If it has the motor on the rear that can be removed it could be a number of sub classes. The sub classes are from the small improvements the engineers made during the course of the models run.
I know that this is clear as mud but that is the basic gist of it.
Billy