What I can see in these pictures looks like a 15-88, although I know nothing about the 15-96 and you appear to. On the 15-91 the rear inspection plate is removed and the motor attaches directly into the machine, I believe gear driven. On the 15-88 the motor sits low behind the column and is held in place with a large screw on the side of the column below the hand wheel (you can see the protrusion in the above picture but I don't know what other models may have that protrusion). The motor on the 15-88 has a small wheel that lines up with the handwheel of the machine and a short belt goes between the two, in place of the treadle belt. There is a rod that attaches to the treadle peddle and goes up to the machine or motor instead of a cord. I don't have a good picture of that part of the machine, but I'm going there next week and will try to get some pictures.