Looks like that glow effect was done by the camera operators increasing the exposure time on the film, making the image brighter. This was made in 1974, a time before backlighting effects were common.

Yeah, it's still a technique used today (over-exposure, 露出オーバー), like when someone comes out of a dark room/cave, or its an establishing shot for the beach in the summer-episode or something. These days its digitally done of course.
There's several books on the subject of traditional compositioning in japanime, and it's so interesting.