Hi Berkley! Thanks for the additional photos. Are there any threads in the hole? That would pretty much confirm that this gun was percussion at some point in it's life. But I don't believe anyone would fire this gun with a vent hole that large.
The blow-back would be dangerous. Sometimes vent holes were made a little larger so that while loading powder in the barrel, a small portion of the powder would fall into the pan area with the frizzen closed. This would save the extra step of having to prime the pan. But the size and position of this hole look the right size for a percussion drum. There were a number of Baker pattern locks that were percussion (or factory converted). And this may be one of those guns that utilized that lock? Being re-converted to flintlock would not be surprising since often percussion caps would be hard to find in the area, and expensive. Again, if there are threads in the hole, it was probably percussion. It may have had a vent liner installed during the re-conversion to flintlock. The most common material for vent liners during the period was gold (low grade) and may have been removed and sold when the gun was no longer being used. The interesting thing is that the barrel and lock patina look similar. And the lock that is on it now, is one of the most common and correct.
Of course I'm just speculating on some of these comments. And in any case, it does not distract from the overall appearance of the gun. It's a nice piece in really nice condition. Rick.