I picked this up at the local ReStore, a worthy 50 cent purchase. It's missing the wedge-shaped block that would go over the hanger bolt at one end to clamp the sandpaper, which won't be hard to make (item #1,475 on the list).
It's labeled "CCC" - I find myself wondering if this was the California Clever Corporation, or whether it was something the Civilian Conservation Corps folk made for their projects.