Flexion extruder BLTouch mount

This is a part designed to mount a BLTouch z-probe to the two M3 threaded holes found on the **left** side of the Flexion extruder coldbar. It is designed to minimize the (already large) x-offset from the primary nozzle to the probe tip, and properly position the probe in z per the most recent BLTouch smart docs. There is a notch cutout of the top/back of the part to allow clearance of the extruder cleaning brush.

It's designed for and tested with the Flexion dual kit for R1 clones; I don't have CAD files for the single or e3d versions to verify compatibility. Use M3x4 or M3x5 screws to attach - any longer and you risk running into the threading for the holes that mount the bar to the carriage.

The current version is designed for use on the left side of the cold bar - the tapped holes are machined such that the holes on the left and right ends are at different heights from each other, but remain at the same height whichever way you flip the bar for installation. I may design a