Well, I will not pretend to be an expert on English Flintlocks converted to percussion (did not even get to stay at a Holiday Inn Express last night)
HOWEVER- possible that Sutherland was the maker of the lock, not the gun. It was not uncommon for gunmakers to purchase locks, or even to sub-contract the manufacture of locks. And I did run across a reference to a pair of Sutherlands- either brothers or father/son, that were lockmakers. Ramsay and Richard (R&R Sutherland) that had contracts for locks as far back as 1809.
In my VERY limited experience with British military arms, since they were the property of the crown, they were usually marked with the initials of the monarch- such as GR (for George Rex) along with the British Broad Arrow mark. The brass support on the side opposite the lock also does not appear military in nature.
IF the nipple has the correct thread, one fairly safe means of removing a stuck ball and charge (altho messy) is to replace the nipple with a Zerk grease fitting- attach a grease gun, and pump. Grease pushes charge and ball out of the bore, leaving you a bore full of grease (gentle heat will make that flow)
Hope someone else can offer more help- but congrats on a nice bit of history.