Out of the box your lovely Rapax (M1, M2 or M3) shoots semi and full auto depend on the position of the fire selector. To lock it to semi only (DMR mode) you are supposed to swap the positions of the left and right fire selectors undoing the little hex screws that hold them in place. Good idea, however, if you try to do that you will find out that they do not fit well on the opposite side due to a slight design overlook.
All you have to do is to replace left selector with the new one and that will turn your gun into a DMR as it will prevent from turning the selector to full auto position. No need to replace the right selector.

The material is 3D printed plastic, black.

Note to eBay:
This item is an accessory for a toys airsoft BB Guns and is allowed to be listed. 
It's NOT a firearm part or accessory - Please don't end this listing in error.