These are 1mm thick aluminum brackets that have a 3/8" hole and a 5/32" hole on one side and a slot that accomodates 6-32 (or just about M4) hardware on the other side. You use these in conjuction with a 6-32 (or M4) bolt and nut or front panel component (up to 3/8" shaft) to hold you PC board to your module's front plate.
You can position the face plate components so that two of these can mount a PC board. You have some latitude regarding placement because the slot in the one side permits 0.6" (15.24 mm) of sideways movement for your mounting nut and bolt.