This is a family-friendly board, so I hesitate to say.
It's a "Belt Vibrator," but if you Google it, BE PREPARED FOR OTHER FORMS OF "VIBRATOR" IMAGES TO POP UP.
I googled “antique fat melting device” and behold, it was called the Mueller Exercise Belt.
Believe it or watch: https://www.youtube.com/watch?v=Iet33fqk9OE