Note that the 22 CM measurement converts to 8.66" or very similar to the British 9" point. In older muzzle loader days bore was measured at the muzzle. With breech loaders & choke boring this point was determined as the point of measurement. Burrard mentioned that many British guns were "Enlarged" forward of this popint in regulating chokes for a given load at the factory, when New. In efect a Looooong jug choke.
If that were my gun unless I were planning on selling it in Emgland I would just shoot it with appropriate laods, not bother with the expense & trouble of a re-proof, though I see no reason based on the bore & wall they would not do so & mark for the new bore size.
Note also the 12 in a diamond simply denotes the nominal chamber of the gun & not the "Proofed" bore size. If British there should be a non-enclosed mark for bore size as 12 or 12/1 etc. If Belgian, German etc will have a mm mark for bore as 18.5 etc.
