Z-knob stop for MakerGear M2 dual

This is a second attempt to create a useful locking mechanism for the MakerGear M2's Z-axis knob. The first version, http://www.thingiverse.com/thing:873378, does not allow the M2 dual extruder carriage to move all the way to the left. This version stays clear of the extruder carriage in all positions while keeping the feature of releasing itself if the Z stage is homed while the lock is still in place. It has the additional feature that the lock handle can be placed upside-down on the cap an