Hi, I believe the 91A and 91B connecting blocks were used by the ComKey 416 (4A communication system) to supply the T&R to the Primary set (2 CO lines per jack). The connecting blocks are not compatible with 1A2 systems for 4A speakerphones. Check out the ComKey BSP 518-450-106. There are other connecting blocks, but I don't recall any that would bring out the T&R and A Lead pairs.
Remember, you need 2 pairs to your single line set, the T&R and A Lead. The A lead pair will cause the KTU to show the line in use, and light the lamps on any other phones. Without this, if you hangup a keyset that was on the line, the line will go on hold when the single line set picks up IF you don't have A lead control (if the single line set was pick-upped while the keyset was off-hook), or if the keyset puts the line on hold and then is picked up by the single line set, the Hold will not release and there will be a poor voice connection.
There are several methods to do what you want, some more elegant than others! Here is one way: 1. Your single line set has to modified to provide A Lead control as the first step in any method*. 2. Then I would connect the 2 pairs from the single line set to a modified amphenol plug (there are screw terminals for the 1st 2 pairs corresponding to T&R & A-Lead). 3. I would get a CPC (Connector-Plug-Connector) adapter and plug your nearby keyset in one of the connectors, the single line set in the other connector and the cable from the KS into the Plug. Then, when you pickup the single line phone, the corresponding lamp lights on all keysets; or if any keyset places the line on Hold, then picking up the single line set removes the Hold and the lamp is lit steady on all keysets. Hope this helps.
(*Others on this forum posted the conversion steps or alternate using a line sensing relay)
