> using the volt meter between pin 12 and the lamp connections on the terminal strip

..oh, and check pin 37 the same way too. Pin 12 and pin 37 on the amp connector should
run all the way to the lamp socket for line 4, IIRC. Here's the schematic for amphenol #1
on the call director 2636 (I couldn't find a wiring diagram for your 2682a offhand):

[Linked Image from seriss.com]

According to the wiring diagram, these wires don't make a stop on the terminal board,
the O-BK pair goes straight from the amp connector to the lamps.

However, in your photo #3, I /think/ I see the O-BK wire going from the button strip
to the terminal board (screw lug #29?) And I don't see the BK-O wire in the photo
(perhaps it's obscured). Anyway, might be some special wiring in your phone that
needs fixing.. but I'd wait until someone comes up with the actual 2682a wiring diagram,
as it might have different wiring involving the terminal board. If you have one you found,
post it and I can try to read it for you if you can't.