Seen plenty of them. Both drum and disc. Required in the early days of automatic transmissions and PTO's. I don't remember any of the drums using a backing plate. I don't think it was necessary. If I didn't throw it away because I was tired of tripping over it, and you wouldn't mind doing a disc version, I have a Mico caliper designed specifically for that purpose. Cable operated. Yours for the price of nothing. I think I know exactly where I left it.
BTW, the '46 International parade truck I maintain, uses an outside band/drum clamp for a parking brake.
