This thing allows you to use 1/4" all-thread lead screws on the Prusa without modifying the X-ends. It adapts a standard SAE 1/4" nut to the 5/16" trap on the standard Prusa X-ends.
I am switching to 1/4" lead screws on my new Prusa i2, since they are more flexible (less able to make the frame wobble when out of alignment), and they give slightly higher Z precision (20 threads per inch instead of 18). Just press an SAE standard 1/4" nut into the bottom of this, and you are all set.
This, coupled with a bottom support bearing (thing:19313) and a flexible motor shaft coupling (thing:29879) allow you to fully support the weight of the X-axis on bearings at the bottom of the printer without over-constraining the Z-axis, instead of hanging all that weight of the X-axis from the Z motor shafts as in the standard Prusa design.
This is fully parametric. The STL file provided has the following parameters:
quarter_nut_dia = 12.83;
quarter_nut_h = 5.56;
five_sixteenths_nut_dia = 14.66;
five_sixteenths_nut_h = 6.75;
adapter_dia = 16; //
adapter_h = 10; // height of the cylindrical section enclosing the 1/4" nut
rod_dia=25.4/4; // diameter of threaded rod
wiggle = 0.1; // fudge factor
layer_h = 0.25; // this is your layer height for the single layer to support the through-hole