I don't see any British proof marks on that barrel. Remember the British proof house loves to vandalize good clean metal by stamping every square inch of the cylinder.
I wonder if they are seeing a bushed breech block with reduce striker. Although if the gun has been fired there should be "wear" marks from the cartridge head and off set for .22. I wonder even more if they know how to get the breech block out to look.
British mark for .22 is .220 not .218.
The action looks like and old British commercial action--perhaps that is where they are getting the British proofs.
And if you like the price on the 12/15, I'll sell you mine and go buy two more.
