On a 1a2 system it was the "a" lead on each line that activated the lamp on all phones with that line appearance. So when you used a single line set on a 1a2 system that set needed to have the second pair used for the "a" lead for that phone to activate the lamp on all the other phones with that line appearance.

on some centrex applications we would use a separate a lead control device to light the light on a console or bee hive light arrangement to give a busy visual.
Mark

Reliable Telephone Service