@TouchToneTommy.. oh, good point -- that's interesting. Looking at the wiring diagram for the 2565GK:

[Linked Image from seriss.com]

..I highlighted the violet pairs in violet and the signals they affect in cool blue.

I can see the violet pairs get involved with various important circuit paths (highlighted in blue), including all the T/R pairs on the phone side of the pickup keys (PU) on 23 (G-V) and 24 (BR-V), and through the dial button pad on 40 (V-G).

Looks like there's also stuff going on with the A leads and ground on 50 (V-S) and 25 (S-V) via the line switch and pickup keys.

At the head of 502-543-403, in section 2.05 it mentions this:

Quote
2.05 When a 2565GK (MD) or 2565GKM telephone set is not used as a speakerphone set and is multipled with any other set furnishing speakerphone feature, speakerphone leads must be disconnected, insulated, and stored either at the telephone set or at the multipling point. If not disconnected, the speakerphone leads will provide a common path between the circuits of the multipled telephone sets. The leads to be removed are as follows:
T1 (V-G), R1 (G-V), IT (V-BR), IR (BR-V), AG (V-S), and LK (S- V).
Speakerphone connections are shown in Section 512-720 -405 .

So I'm guessing those modifications to the inside of the 2565 phone sets might prevent problems with a C-P-C bridge?