"Welcome to a new and automatic world of picture-taking ! With automatic Focus-Matic for instant distance setting, electric drive for automatic film winding, and completely automatic electric eye and flash, your camera takes all the effort out of fine pictures." (from Introduction section in manual instructions of Bell & Howell Autoload 342)
Bell & Howell Autoload 342 was a 126 film camera made by Canon, but marketed under Bell & Howell for North American market in 1969. Motor for film advancing and electric eye for exposure control are not so complicated in achieving such automation, but it is quite difficult to imagine how an auto-focus mechanism worked in 60's.
Focus-Matic, the operation of this auto-focus mechanism was not based on electronics (I tested the Focus-Matic function without using batteries). Instead, it is mechanical. I guess, it is something like combining bubble level and some mathematics of tangent.
Assumming that the photographer and subject is approximately on the same level (say, the height is about 5 feet - 6 feet), the angle that the camera is inclined to see the subject base, is related to the distance between photographer and subject. Please refer to the below scanned pages from the instruction manual for the detail steps in focus.
By the push of the Focus-Matic lever, the subject distance will be shown up automatically in the distance scale and the lens focus will be set accordingly.
