755A Crossbar PBX wiring

[5415551212]

Greetings everyone I have been doing some reading on the 755A Crossbar PBX, and understand its a interesting PBX in that its a hybrid of a key system and a mini crossbar switch. The key phones 566FB 566MB or 566MD that were exclusive to the 755A worked off fewer pairs than the 1A1 or 1A2 key systems, but it appears 1A2 phones could be adapted to work off the 755.

Here is a video by Howard Harte (the fellow collector who reverse-engineered Millennium Manager) showing off his 755A:
https://www.youtube.com/watch?v=Fxt21lnzOqg

I stumbled on this old thread on fellow member Erics 755 (with a video) showing it at his museum.
https://www.classicrotaryphones.com/forum/index.php?topic=16632.msg173926#msg173926

I am still trying to wrap my head around how the phones were wired to the 755, so there there only one tip / ring pair to each of the phones and the line keys were just controlling "A" leads to relays in the 755 PBX?  The relays in the 755 then selected a trunk line to connect the main pair to?  Or was there also the intercom pair?
Cheers all