Marvelous little rifle, Buckstix!
That chambering could have been a nightmare, seems like you have risen to the challenge!

I have a near-identical rook rifle retailed by W.P.Jones and wearing Birmingham proofs, notwithstanding its obvious Belgian roots.
It has an Anson push-rod fore-end latch, rather than the wedge fastening of your example.
Mine is chambered for the original .22 Long, and seems quite happy shooting standard velocity .22 LR cartridges.

Joke is, the .22 is specifically excluded from Rook Rifle matches in our Big Game Rifle Club National Rules! Damn!!