We have *three* makers to explain: Tryon, Maslin, and W.G. Chance.
It's not just a "scroll guard" question. The OP's trigger plate including its exact shape and engraving, the pointed tang, the buttplate, the escutcheon instead of sideplate, these are all straight-up English sporter. The barrel is not like a London rifle/gun, but this is not a London gun anyway. It is a gun with all English hardware on an English-style stock, marked with the name of a company that imported Birmingham guns and parts. The importer was also a builder who made rifles in a distinct American style that looks nothing like this.
Any explanation needs to cover the four Maslin guns I posted. Supposedly from one guy, two are obviously American longrifles, and the other two are entirely English in hardware and stock design, including the checkered wrist. I don't think anyone would claim the same guy made both. Either Maslin imported the English guns, or he imported an unheard-of English gunsmith to Baltimore or Phila. The simplest answer is that he imported Birmingham guns. Maslin was an exact contemporary of Tryon and moved to Phila. almost exactly when Tryon formally set up a business importing Birmingham guns and watches etc. Again, this suggests that Birmingham guns were being imported for some part of the American market.
We should also explain this rifle. It is of the same puzzle. It is marked W.G. Chance, Birmingham.
Chance had a NYC office and forwarded Birmingham goods to St. Louis. He sold to the American Fur Company.
This rifle has the same typically English TG, trigger plate, tang, escutcheon, checkered wrist. The stock is walnut but has far more drop than any other English sporter I've seen. The drop looks American. But the crescent plate seems to be added. The stock has been thinned by dishing out the left side of the butt to fit a narrower plate. I think this rifle originally had an English-style wide butt with a flat plate. But I think only the left side was dished in order to keep the patchbox, which presumably was original.
This is undoubtedly a Birmingham rifle. But it has un-English stock drop and that odd patchbox. Perhaps some Birmingham manufacturers tried to accomodate what they thought Americans in the fur trade wanted. That is one possible explaination for the bbl on KMac's gun.
At any rate, this is a flint proto-plains-rifle that was built in Birmingham and doesn't look entirely English.
When you look at Tryon, Maslin, and WG Chance together, a picture emerges of Birmingham guns being imported for the trade and influencing what American rifles became in the west.