Ah, if you look closely a short distance in front of the bolster, I'll bet that there is a seam - this is because your plug is actually the breech of the gun, forged as a unit. The French did this on their early Percussion muskets (the M1840) and decided the breeches were too expensive to forge. Modern repop CW guns often do this but the breeches are cast. Hoyt should be able to get the breech off without too much problem. But be aware that some Belgian guns have reverse threading..... (Righty loosy, lefty tighty.) That may not be the case here, just something to watch out for.