Here's a thought for you. As CO lines go into the KSU on the left side, can you trace them? Maybe one line does not go to the Telco D-mark but goes off someplace else, you could try to press the associated CO button on a phone and see what happens. Another idea is to try paging all phones one at a time. There is a chance they taped off of a speaker in a phone to connect the page speakers/amp to the system.
Just a couple of ideas to look at.

Stephanie

Do the job right the first time.