Simon:
The A lead is just a ground that is switched by the hookswitch and hold button inside the set. I guess they just needed two terms for either end of what is effectively a loop.
1A1 and 1A2, or GTE's 10A2 were just nomenclature for the family of hardware, just like 5ESS. 1A1 was the first generation of modular key systems made by Western Electric and when a major change came about (plug-in cards instead of hard wired KTU's(relay units)), it became 1A2. 5ESS is simply the fifth generation of AT&T's electronic switching system, the first one obviously being 1. By the way, there are still plenty of #1ESS switches in use (actually 1A's) and 4's in North America.
Oh, and yes, LG is Lamp Ground. Ding ding ding, tell him what he won, Johnny!
