Jim,
You're right and you're wrong. Mechanically, an OPX and OPS are identical. Sure, you don't order an OPX when you want an OPS, but the hardware used to transport them is the same.
So, in Comdial parlance, an OPX card can be used for both on-premise and off-premise single line extensions. It that case, the only difference is protection and voltage.
LkEErie