Hi DMoore......welcome to TFF.
Seeing as your pistol has a percussion lock, that dates from about 1830 to 1870......or.....it could be an earlier flintlock pistol that was converted to percussion in that timeframe.
This may, or may not, be a British made pistol. It could also be an American gun. Ketland & Co. of London, England, had an American branch, Thomas and John Ketland of Philadelphia, which imported and sold many thousands of flint and percussion locks.
And, unfortunately, the "Ketland" marking doesn't tell us who made your gun. It only tells us that it has a Ketland lock.
Here's a bit more info for you: http://www.ambroseantiques.com/fpistols.htm