This is derived from nopheads Mendel Pulley with captive M3 nut, but has been redrawn in openscad, and added more nuts for extra grip.
This part is also part of my OverlapStrap project which is licenced under GPLv3 see: http://www.gilesbathgate.com/OverlapStrap/
or checkout using
git clone http://www.gilesbathgate.com/OverlapStrap.git
If you want to render/modify this using openscad then you will need to take the version of openscad from svn >= r545 or Release >= openscad-2010.05 since there was a bug "normalizing zero-length normals" that prevented the resulting stl from being opened.