Getting audio from a Bell & Howell Filmosound 8 Projector
The projector you have is for use with the Bell and Howell model 450 cassette recorder. This projector has no optical or magnetic means of playing or recording sound. IN this system, which is a "Double Sound System", the sound is recorded on the 450 player/recorder via a special cable that goes between the camera and the recorder. The film itself has no magnetic or optical sound track on it. The recorder records both the audio from a microphone and sync information from the camera, hence the special cable running between camera and recorder. After the film is processed and returned, it threaded into the projector and then the cassette player/recorder is connected by means of another special cable to the projector, via the 2 jacks pointed out in your photo. The jack questioned is a size 3/32.
This special cable is used to deliver sync information that was originally recorded on the tape to the projector for lip sync sound reproduction