It's a mystery to me why when the U.K. adopted the new sockets and wiring method in the early 1980s that they didn't take the oppotunity to dispense with the third "ringing" wire. Clearly this third wire continued in Ireland as well, although as you've told us that there were some 500-style sets in use, it clearly wasn't needed for all phones.
Another use for a master jack which can cut off the feed to those downstream is an auto-dialer on an alarm system.
