This is a shaft mount for a Bowden style extruder for the Printrbot Jr. There are two files here.
The PrintrbotJrBowdenToExtruderShaftMount.stl includes the features necessary to attach a pneufit adapter and the other doesn't. I had already printed and glued my pneufit adapter into my thing 90335, and didn't want to take it out and glue it into a new one. The other file just has a hole for the adapter to fit through.
You can mount the extruder in either direction, but it would probably be better to mount it in the opposite way as shown in the picture to make it easier to change/add filament. Mine is an older version that is too short to mount it the other way, and I haven't yet printed out the updated one.