89, 90, and 91 are the same with different power source
89 is treadle, usually has spoked flywheel. Will work fine with motor too.
90 is same as 89 with solid flywheel and external motor.
91 is same basic machine with the built on ( not in) potted motor with gears
Most of the different 15 models are heavier duty industrial. There weren't all that many versions of the domestic model. I think that is what leads to sellers thinking their 15 is industrial when they really have a domestic model. Some where the same on the outside with heavy duty insides.