Monty,
It looks a very nice gun!
Early as well. V nice engraving. I like it.
Two or three things come to mind;
1, Lightly struck and worn away,
2, Barrel shortened from breech at one time, and proofs lost. (Highly unlikely in this case, usually happens when a gun is percussioned and new breech fitted)
3, Some English guns seem to just not have had proof marks for some reason. we see best guns without proofs at times.
Also bear in mind that only London required proof at one time, though most makers had barrels proved either private or at the Tower.
Again, a very nice gun.
R.