Like Merritt sez--the violet/slate pair is a PF phone. All of the others on the 66 block are for the 12 phones (2pr each). Besides the 25pr amphenol, the six mod ports for the C.O.s, the six screw terminals for loud bells and a phone alarm--there are no other terminals present on the KSU that I know of.

Back in the early 90's when we had a customer that wanted MOH, we "upgraded" him to the ModKey 16.

If there is an aftermarket gizmo, or a way to solder something on to the backplane---I'm sure a ModKey 16 would be less trouble---but I may be wrong. Good Luck.