Some projector names from about then include Bell and Howell and Eumig, at least in 16mm and 8mm film.
In TV studios, our two Bell and Howells and the slide drum were on a multiplex chain and in order to get from one reel to another, the operator had to do a 'douse and show'.
Presumably the picture theatres had a similar changeover system??
When old movies are shown on telly, you still see the cue dots go through - at 8 seconds before, when the other projector would be started, and then at change over time when you put the lights out on projector 1 and lit up projector 2, and you hoped no-one saw any gaps in between! It was quite nerve wracking!
But that was back in the days of steam radio!
Robyn ex-NZBC WNTV1 Telecine Operator.