This info might not be a bad thing to sticky, or transfer it into the nightmare sticky. I need a feed plug for my aluminum LB, so I took the dimensions of the Co2 plug since the threads are the same.
This pic shows the 12g cup on the end of the plug.
As for the feed plug, I do not have one to measure. Although from the few pics I have seen, the knurling looks to be no more than 3/16 inch in length. The thread length should be a bit shorter than the feed plug threads as well. Maybe 1/4 in to 5/16. That is a judgement based on screwing my co2 plug into the feed plug threads. The threads bottom out in the gun leaving the gap visible between the threads and the knurling.
*** On the outside chance that these threads are actually metric, The closest I could get would be 22.50mm by 1.25 thread pitch. The thread pitch gauge seemed to mate up slightly better with the 20 tpi though. Thread diameters were measured with a digital micrometer. To 7/8 in threading measured up to .880 inch. 7/8 = .875. The metric equivalent was about the same off from the assumed size. Oh, and I am an auto tech, not a machinist