Possible Lend-Lease to UK in WWII. In 1940 they were desperate for ANY kinds of weapons- some Home Guards were issued bayonets welded to lengths of pipe as pikes. Many Americans sent privately-owned guns; this may be one. If so, this one is lucky to have made it back- the British, in spite of promises to return them, destroyed millions of "evil" weapons after the war.It did some traveling....to the UK and back apparently.
The mark says .455 MK VI. As far as any BB marking with numbers I don't see it anyplace and I didn't see a c with an arrow at all either. The only other marks I see is a little crown type thing on it's side with a curved line to the left. That same marking is on every cylinder.