Can you post good clear photos of the face of the cylinder and also the muzzle/ rifling?
Let’s establish a baseline first to make sure it hasn’t been caliber altered.

I’m seeing what looks like sleeves in the cylinder. Also that visible line towards the end of the barrel supports that the barrel was relined as well. My guess is this gun was caliber altered.

The oval with crown over it stamped on the cylinder is a Belgian black powder proof mark.

Murph