It's an end stop mount for the z axis. I finally got sick of the original and good enough at design to throw this together. The holes are designed to be screwed into, relieving the need for nuts. It fits firmly against the acrylic case and keeps the end stop pretty stable. All you could ever ask for in a 5 cent piece of plastic.
I designed it for the lowered height of the e3d V6 hot end. I had to add a nut to the head of the actuator to keep the spring tight, and it works well.
The actuator is an m5 x 25.
After printing, run some screws into the holes first to make threads.Mount the end stop, then mount it to the printer.
Now to set it up. Tighten down your bed springs. Level the bed based off the lowest spring height with an actual level tool. Then move the z axis to 0,0 and 0.1 above bed and measure with a gauge or paper. Move the hot end to 190,0 and twist the guide rods on the right hand side until the hot end comes down to 0.1 above the bed. This is going to make sure the carriage is level with the bed. If your bed springs are very tight I've found they won't tend to loosen up on you, and you get extra height out of the deal.
Get the mount in rough position so that it's activated in its current spot. Then home all axis, measure off 0,0 at 0.1 mm above the bed and fiddle with the set screw on the x axis until you get it where you want it.