I briefly had one of these in .45 and the breech plug wasn't secured but rather when the action was opened, the breech plug was simply blocked from backing out all of the way by the receiver. Otherwise it slide backwards up to point that was about 1/2 of it's length.

To remove it would simply require removing the barrel from the receiver and pushing the breech plug out of the barrel breech. It wasn't threaded and I don't remember there being an O-ring. The breech plug itself was basically just a sleeve.
The firing pin did have trouble making contact with the primed nipple so I shimmed the nipple with a washer which cured the ignition problem.