Oh no... I'm not sure I can live up to that introduction!
What we really need are some pictures because these marks look very similar and are easily mistaken when going by a simple description. However, I have a Ketland proofed pistol on my desk at the moment. It has two marks, Identical, a crown over crossed scepters. The 1813 Birmingham proof marks have the same symbol but with "V" added for the viewing and "B P C" for the final proof. However, there are fake Birmingham proof marks too, seen on Belgian made barrels, and distinguishing them can be a problem if there is substantial wear involved.
The date of the establishment of Ketland's private proof house is problematical. Dates as early as the 1750s have been mentioned but I can find absolutely no proof to suggest it could be that early. I would say that Feltwad's dates of 1766-1813 are far more likely to be appropriate. Thomas Ketland Sr. married in 1761. I suspect he was about 22 or 23 at the time meaning he was born around 1737. (This date has appeared in print but again, I am still looking for the actual record of his birth) Although mentioned in der Neue Stockel and in the late Kit Ravenshear's article on the Ketlands, I have found absolutely no proof that his father (who hasn't yet been absolutely identified) was even involved in the gun trade. He certainly isn't mentioned in the first Birmingham City Directory of 1767. This is where the 1766 date comes from. It could be a little earlier but I doubt much earlier.
As to your gun, again pictures would be helpful but there is no reason to presume it wasn't originally a 1/2 stock. I own the only known Ketland wholesale price list (dating from the Summer of 1812) and it specifically offers "1/2 stock same as whole stock". If your gun is a Ketland product (as opposed to a gun with a Ketland lock) it is almost certainly much earlier than 1830. By then the family was all but gone from the trade. Of the two Ketland firms, the W. Ketland firm went into bankruptcy in 1825 and only one of the four sons of Thomas Ketland Sr. was still alive and he is listed as a wine & spirits merchant. He may have retained some involvement in the gun trade but 1830 is about the last gasp of the Ketlands.
You say its a Ketland lock but how exactly is it marked? Keep in mind that the Ketlands were "gunmakers" nearly in the sense that Sears Roebuck was in the 1920s. There are also period "fake" Ketland locks. This family controlled such a large portion of the American market that their name was widely copied. This was apparently not illegal or at least so difficult to prosecute that the only recourse was to publish advertisements in newspapers decrying the fakes. Both Ketland firms did this in American newspapers.
Other the the progenitor of the family, Thomas Sr., I doubt that any of of the Ketlands ever so much as handled a file although I believe that they throughly understood the complicated workings of the Birmingham Arms trade.
In any case, I have a lot of new information on them and am currently preparing an article for Man at Arms...in fact I'm in the office today struggling with the transcription of Thomas Ketland's will!
JV Puleo
Well, I realized I had a picture of these proofs but I don't see how to include it. I did know how to do this in the past but it must have changed when the format changed and how it is now done isn't obvious to me.