This collection of parts includes what I consider to be the latest, best designs of printed parts for the X-axis for any Prusa i3 variant, including the P3Steel and the Prusa i3 Twelvepro.
Some of them I remixed, some are just as I found them here on Thingiverse or GitHub, and some I created in OpenSCAD.
The X-carriage is my earlier thing #586636.
The Open Hardware Knurled Knob used to adjust the tension of the belt is my earlier thing #600955.
The 623ZZ pulley is my earlier thing #602435.
I am still researching the proper attribution for some of these parts (see http://reprap.org/wiki/Genealogy_/_Archeology_of_the_Prusa_i3_X_Motor_End), what is certain is that they are all derived from Josef Prusa's original designs which were licensed under the GPLV3, except perhaps for the pulley which I wrote from scratch in OpenSCAD, but that is trivial code; and the knurled knob, derived from other things found here on Thingiverse (with proper attribution).
Note that both the X-carriage and the X idler end pulley are for a GT2 belt.
All these parts print easily. I have been printing them in PLA with 40% infill, 0.6mm walls and top/bottom layers.
The X idler end pulley requires 2 x 623ZZ bearings.
The reinforced X carriage uses 4 x LM8UU linear bearings instead of the usual 3 bearings.