@ProfShadow: Ah, thanks for the dead link notice.. I'll need to make a page for my 551a, as I don't already have one. For now I put a stand-in page so it doesn't 404 at least.

BTW, I've updated the webpage to include a better circuit description, and I also added the board's .pcb file (used to layout the board and have it printed). See the page for details if you're interested in that.

Relevant to the subject of this thread, I used that same software to make a small pair of boards that converts 1A2 50 pin rj-21 into (3) rj-45 cables and back at the other end, so one doesn't need a full 25 pair cable to do long runs between the KSU and phones, and without punch blocks. With one RJ-45 cable between the boards, one can get Line 1 (TRLA) + ringing. With two cables that adds L2/L3 (TRLA), and with 3 cables adds L5 and buzzer (e.g. intercom). One can leave out the middle wire if one just wants L1 and Intercom with ring+buzz. There's two boards needed, one at either end of the RJ-45 cables, with proper male vs. female rj-21's for each pair. Here's the "phone end" of that board:
[Linked Image from seriss.com]
The "KSU end" looks similar, but with a different sexed connector, and thus the trace layout is a bit different. I'll probably make a separate page for this little pair of boards at some point.