Interesting. I had a debate last year about the single QF INS. Its good to see that someone else remembers it. It was 1971 as I worked on the project as a LAME and went directly to FE training from that.
IF the 2 INS were not fitted by 1974 it was close. In 74 I took my wife on her first staff travel trip and 2 of the most senior Navs were on it as a retirement trip so I guess they would be the ones to turn out the lights.