I know perfectly well how a standard threaded autococker feed neck unscrews. After building four autocockers, replacing a threaded neck for a friend, and having seen several be replaced at the local pro shop, I hardly need it diagramed for me. There have been jobs done by Docs Machine where bodies have been tapped for different necks though. If you buy or build bodies without the holes drilled, or if you simply cover the hole (I believe in one case, (wasn't on a cocker, but could easily be done to just about any marker), a feedneck hole was silver-soldered back together, and a new one re-drilled), and re-drill and re-thread it (and re-finish the body, of course), you could put basically any neck you wanted to in there, but like I said, you'd need a custom machine shop to do it, unless you happen to have a spare multi-thousand dollar machine shop in your garage.