The 90 has the external motor, and the smaller solid flywheel. I think you can swap flywheels on them.
The 88 came treadle or electric, and has the larger spoked flywheel. the 89 is the same with the handcrank attached.
I had an 88 that came in a treadle cabinet. I put a motor on it and it was one of the nicest sewing 15's I've had. It went to a budding seamstress that needed a machine for sewing a denim quilt.