From Tony Busers original thing:

This is a grooved insulator retainer/mount for a MakerBot plastruder. This is for MakerGear makergear.com/products/nozzle style grooved insulators. (Pictured here is one I got from Brian Reifsnyder reprapstores.com/catalog/extruder-parts/machined-parts/tb-hybrid1g who also sells them and should have the same specs)

The idea is with a grooved insulator, you don't need the big washer to attach your barrel. You just slide it into the slot then slide the horizontal bolt to keep it in place. The bolt may not even be required depending on how tightly it fits.
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This derivative uses the other 2 bolt holes (the symetrical ones). The most up to date version of this can be found at http://gitorious.org/hacks/things

From Tony Busers original thing:

This requires some pretty tight tolerances so you may need to modify the barrel_diameter and barrel_inset_diameter variables in the openscad script. That or carve the slots out a bit with a utility knife or sand it.

You'll need 2 M3X15 bolts and optionally an M4X45 bolt and M4 nut.