I noticed the same on Hitcocks site. I wonder if it might be included with the pivot pin part I think bolts through it. Hitchcocks might know, and would likely be curious about your situation.
Pex tubing - people use it as diy replacements for door jam striker bushings in the auto world, it sounds like it is up to task. The 1/2” ID tubing should be fairly close to 12mm and about the right OD as well. About 5 bucks to try.
There are lots of online options for custom parts there days. This guy might could help with a delrin replacement which probably is superior.
I can make you custom delrin/ acetal bushings(headless, or with flange). (1) CUSTOM DELRIN Plastic bushing for pbsch929 (#192456530266). (4) CUSTOM DELRIN/ACETAL Plastic bushings, for smag2977 (#192467863146).
www.ebay.com
And a business which makes lots of bronze bushings and can do some custom sizing for you. They offer 7/16” ID along with the 1/2” below, actually scores of sizes so one should be quite close and not need much touchup
With over 900 different sizes available for immediate shipment, Sleeve Bearings provide and off-the-shelf, economical, and fast solution. They come in both SAE841 (oil impregnated) and C93200 alloys.
bronze-online.com
This is an interesting question and one another of us will surely face, I look forward to its answers.