NJ Quilter,
The sub number (-#) does not affect whether or not the machine head will fit in the cabinet. I does affect whether or
not it is operational in the cabinet. An example is the Singer 15-91. The handwheel on this potted motor model won't
take a treadle belt, so it won't work as a treadle in a treadle cabinet. The 15-88 (treadle model), the 15-89 (the hand crank model) and the 15-90 (the belted motor model) all have motor bosses so all will accept an bolted on motor or hand crank or a treadle belt. If the power source is removed (handcrank or bolted motor or treadle) these three heads are identical.
I can't help much with the 66 sub numbers without a little research. I do know that the sub numbers on the 66 also won't affect whether or not they fit the cabinet. It can make a difference in whether or not the bobbin winder will work on the treadle models or if it has a motor boss to take a hand crank or bolted on motor.
Have you used this feature of Sandman?
http://www.sandman-collectibles.com/...r-machines.htm
I hope I didn't just confuse you more,
Cathy