Probably third wire supervisory signalling where the Ambassador is piggybacked onto a cordless PMBX.
"Ambassador ESS systems can provide earth-loop recall or 'C wire supervisory' signals to host system."
http://www.britishtelephones.com/pmbx2_3.htm
There were still plenty of those PMBXs in use until they were withdrawn in 1994 where the size of the installation did not justify the newer eg Herald systems, and many of them had a Plan 105 (later, Ambassador) piggybacked on for boss-secretary working.