Picture frame hanger
thingiverse
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.
Download Model from thingiverse
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