Jeff: "Ground return" simply means the wire that completes a circuit from a device that had a "hot" lead activating it. For instance, the 10 volt AC lamp supply goes to the lamp, through the lamp, and back to system ground. That's the ground return for that lamp.
Cat6: The 564 predated the 565 by several years.
The "A" lead is one of three leads that are switched when a line key is depressed. (We say "key" rather than "button" because historically, people operated keys, going back into the days of telegraphy. Buttons are those little things that are on a TT dial or hold your shirt together. That's why these things are called "key" systems.)
The line key makes three leads: the Tip, the Ring, and the "A" lead. You know what the T & R do, and the third contact on the line key operates the A lead, which takes A1 ground (present in the tel set on the O/W lead, via the break contact on the HOLD key,) and sends the ground via the switch-hook and the A lead to the line circuit relay associated with that line. This relay can be part of a hard-wired group of relays (as in 1A1 systems) or on a 400-series printed circuit line card (in 1A2 systems.) The line relay operated sends lamp battery to the lamp lead of the line on all sets, and prepares the HOLD circuit. When the sub hangs up, the A lead opens, and the relay falls, releasing the line lamps. If the sub presses the HOLD key, the A ground is released just before the T&R are released, due to the mechanics of the HOLD key/line key linkage, and the line circuit starts the interrupter, switches the lamp lead from steady supply to wink supply, and the line lamp indicates a HELD line by the winking lamp. (120 ipm). On an incoming call, when the sub presses the line key and goes off-hook, the A lead serves as a supervisory lead, tripping the ring, changing the lamp supply from flashing (60 ipm) to steady.
Got it now?
