We had modified Logic 20s with non locking keys and a 20 button addon and the set was called a logic 40 that would work on a 50 pair cable almost 40 lines and you could add to each 40 button set another 20 button module. There was a limit how many you could put on. There was only one talk pair for all the lines and 1 A lead and 1 lamp lead for each line and the grounds were commoned up. It was sort of a concentrator It saved a lot on cabling.
