This is a keyed wall bracket. I think similar to a “floating” wall bracket. Seems sturdy enough. There are three layers of 3mm baltic plywood per bracket glued together for rigidity and strength. Need to counter sink the outer hole to be flush so the main arm bracket slides without catching. Too lazy to spackle and sand the layers so I’ll just live with the visible banding of the layers. Painting them black hides it pretty well.
I scanned the pattern from a 2003 Architectural Details book (Vintage Wood Works) that I found at a used book store. Then used Inkscape’s Trace Path feature. What a lifesaver!