These are clamps for mounting power supply in a Prusa i2

The mount system is intended to be used with a swithing power supply 50mm thick like this one:

http://bit.ly/18XYyUx

It consists of three brackets bolted to threaded holes in power supply. Many of these Chinese power supplies are similar in design.

Additionally you will need:

3 - M4x10 bolts (l guess longer bolts will work too, but I think you could touch some electrical component inside)