This 1970s Synchronex Super 8 movie camera once belonged to my grandfather. It still works, and I recently used it to shoot some film. It came with this cassette deck that recorded audio synchronized with the picture. The cassette would then be sent in with the film for processing, and the audio track would be copied onto a magnetic strip at the edge of the film. Unfortunately, no film-processing company still offers this service. It’s still a good Super 8 camera though!
That’s a pretty cool (if convoluted) way of getting the audio track on an 8mm film, but this would allow better miking/recording of the audio than relying on the condenser microphone or ‘overhead’ small boom microphone other movie cameras had next to the lens.