Picture frame hanger

They are picture frame hangers.

You have to drill a 22 mm hole in the back of the frame and glue the hanger to it.

You can mount it either one way, with the 3 recesses at the top, or the other way, with the smooth part at the top, depending on how you want to adjust the picture to make it stand straight.

I have put the source in OpenScad in case you want to make your own version.