Scratching the memory banks here, but I seem to remember Viscounts with NATO sockets and BAC1-11s with NATO and RJ55/68 twin sockets. I assembled some of the audio panels as an apprentice.
I made up a lot of splitter cables for private pilots in the mid 1970s as there was a mix of sockets on light a/c. Also modded some aircraft to one or the other, or both.
I think the BN Islander had NATO.
Airlite 62 was the common headset at the time.