I was perusing the Osprey book "Medieval Handgonnes" and the author referenced a very similar barrel in Denmark.
https://samlinger.natmus.dk/thm/asset/43719
He says that there is a makers mark "I S" on the rear of the barrel.
Hmmmmmm ... "IS" mine has "IV" could there be a connection???
Take a look at the rear of the barrel:
The date of the barrel is 1515.
See the prob?
BTW, you find anything further on the barrel I bought from you?