This is a 2 piece swiveling mount for a Grundtal Spotlight, available here:
I needed to defuse the lamp light against the ceiling but I wanted an adjustable angle. The 2 half circles allow 180 degrees of rotation. Currently the fit is snug, and a doubled over twist tie "pin" is keeping alignment.
In the case that over time the fit becomes loose, I will replace the twist tie with a screw and wing nut. As of now, this is not needed.