This is the "0" offset breech, modified to be 7.0mm thick (instead of 7.5mm.) This seems to fit much better between the legs of the Nimble v1.2.
The middle pin is a piece of 3mm brass rod cut to ~7mm with a dremmel. (Dremmel cutting wheels aren't exactly precision instruments.)
For the top/bottom pins, I decided to bore the holes with a 7/64 inch drill bit, and then use M3x14 (threaded) button head screws. I'm not entirely sure why the original model called for using partially threaded screws (unless it was to try and prevent the screw threads from damaging the nimble body.)
This model is letting me use my nimble (and printer) while waiting for shapeway to deliver a proper replacement breech. (Although, I kind of prefer this replacement. Using a screw to lock the breech closed is much more secure than the nylon tabs on the original.)
Supports must be used on the inside, and they can be challenging to remove cleanly.