Summary

For attaching background panels to model layouts and dioramas, among other low-strain uses.
I sized them to attach a 1" x 3" post and 1/4" masonite to the underside of my model train table such that I can easily lift off the scene board for transport or for maintenance.
They need around 1 1/2" vertical clearance to lift off. The socket is tapered so that when assembled it should sit down tight and not wobble, although the pocket brackets are prone to twisting a little if someone pushes the posts to the side.

The inside surfaces that mount to the lumber are flat. Print both pieces with this side facing down so that support isn't needed. I used PLA with mine, in an older Ender 5 printer.