This allows the Raspberry Pi 2,3,4,5 (NOT 1) with passive cooling heatsink case or without a case to be attached to standard 35mm DIN Rails. The adaptor has M2.5 tapped holes to screw to the standard Raspberry Pi 2,3,4 mounting holes on a bare board or if you have cases that also use the Raspberry Pi mounting holes then they can also be screwed on directly.
eBay