Appears to be a hidden cross bolt gun, I can’t see where the bolt dutifully pokes out the left side of the detonation? Unusual style for an English gun.

It looks like a Grouse gun to me. Canvasback will turn you on to a good Canadian gunsmith, he has them all on retainer. Likely due for a strip and clean, at the least, and a repair to that crack on the head.

Good luck.

Best,
Ted