Mike Sandman in Chicago used to make an A-Lead controller. Someone else did too, Melco, maybe? Give them a try. Like Tommy said, you can check ebay.

A 6B 1A KTU will give you partial control. You wire the KTU in series with the circuit. When you go off hook on the single line device, the KTU gives you a short across the A-A1 leads, when you hang up it opens the short. It works fine to give you lamp indication on a key set, but if the key set is on hold, it won't take the line off hold.

An 18D 1A KTU should give you full control, but I don't remember, it's been 30 or 40 years since I've seen one.

I probably have a 6B in the dreaded basement. PM me if you're interested.

Sam

"Where are we going and why are we in this hand basket?"