Sorry to burst the bubble, but this person makes forgeries for ebay.
He sells haquebuts poleguns etc as well.
The fact that this piece is:
-so well preserved after a odd 500 years
-only 10 cm in to the earth with "fresh" leaves covering the piece
-the fact he knows what this is for an "amateur"
-the real Monk's gun is just a curio, not a standard firearm of the time and finding another one in the earth like this is more than curious.....
Still a fun display and interesting if it where in "new" condition.