That is an interesting conversion. I haven't seen the indents left that way in the barrels. Thanks for posting the pictures.
Montana, I wondered about this too, with the indents for the pinfire pins left at the breech of the barrels. However, the gun shoots fine with low pressure 2 1/2 inch shells. No flame escaping at the breech end and plenty of barrel thickness both at breech and muzzle. I suspect the gun has been here in the States since at least the 1940's, but have no way to prove that. Do you know when import or export marks were first required? Also, I don't have a good reference on older British proof marks and I have yet to decipher the ones on this gun.
Thanks for the interest, I think it's a fascinating gun.
Brad