From this information I believe it originally was a .450 with black powder proofs only. At a later date it was bored out to .470NE but it was not re-proofed. Someone simply overstamped the black powder proofs and added non-official "nitro" proofs.

Any .470 proofed in England will have the scimitar nitro proof stamp that came into use with the rules of 1904.

It was a Webley A&W, not the Webley A&W C.

This is the reason your purchaser shied away from it, taking issue with the barrels.

Curl