It seems that hook has been broken. Look at the Berns chronics. We can see the muzzle neb of absoulutly similar shape.
Michael, i remember that You discussed barrel that looks like this cut in half
But I think that it is not early 15 century. I guess it much later
By the way this museum is not so far from Saint-Petersberg. So, It is possibly that I will visit this museum to make some photos of this handgonne