That backing plate you have is the same as what I have on the rear of my M138 Alpina. My hub is the two-piece rear hub (brake drum on opposite side to sprocket) as fitted to Pursangs and Alpinas after about 1973 or 1974 and also to Fronteras.
I suspect that same backing plate you have may also fit the one-piece rear hubs (brake drum and sprocket on the same side) fitted to earlier Pursangs and Alpinas, but not the very early rear drums that have twin brake shoe pivot posts and wider shoes.