I needed a way to visually identify a Pi in a lab with multiple Pis mounted on the wall with DIN rails. Hopefully it comes in handy for others as well.
Once the label holder is pushed through the mounting holes of the Pi, you can lock it in by pushing a flathead screwdriver into the slot on the post to widen it.
This should work for Raspberry Pi Models 2 through 5.
I used these Dymo 30252 Labels (1-1/8" x 3-1/2") but it should work with other labels of similar size.