This is a stop block that fits a Festool track.
It is designed to set starting and stopping points for a track saw.
An M6x12 bolt is used to tighten the block with a corresponding M6 nut trapped in a plastic adapter that goes into the aluminum extrusion.
A knob (made with the brilliant nut knob generator: http://www.thingiverse.com/thing:713300) grabs the hex head of the M6 bolt, making it easy to secure and release the block.
A washer can be added to serve as a spring, between the knob and the block, but this is not mandatory.

The block has a round notch that catches the corner of the base of the saw so there are two models: a start/front block and a stop/rear block.
Typically print one of each block and two knobs plus two nuts.

2017-05-16: Added a start-front symetrical version of the stop as requested ;)