DIN rail (35mm top hat rail) mount for another DIN rail. Used to lift DIN rail elements up to align them better with the top of neighboring MCBs, etc.
With PLA the locking nub just chipped off, so I used PETG instead.
I had to sand off some print artifacts from the slider sides so that it would fit into the main body. Don't sand off the locking nub!
The author hasn't provided the model origin yet.