I'd really like to know what the supposed connection between the tombstone mark - of which there are numerous variations - has to do with the Ketlands. I've never seen anything in the primary documents to suggest this but I have seen plenty of those tombstone marks on guns marketed under a variety of names.
Two British authorities, Brian Godwin and John Evans, are currently working on a paper on the tombstone marks but they assure me that, while they can draw some conclusions, exactly what the mean remains a mystery.
Also, the crossed scepter "Ketland" mark (which is different and does not have the letters "BPC" and "V" included) is also a myth. The mark is actually the Ordnance Private proof mark used on guns proved for the trade at the Ordnance proof house. The Ketland connection is completely mythical, based on a mistake made by HBC Pollard back in the 1920s and repeated endlessly ever since.