Well, I'm at a bit of disadvantage here as I'm out on the ship, and the only reference I have available is the KRA CD of rifles. And I have to admit that I don't have Shroyer/Koons details committed to memory.
I agree that the butt plate is different than Shroyers, and I don't see a lot of Shroyer in the carving either, and it seems Shroyer liked to do that cut where the wrist meets the butt stock, which this rifle is missing.
But if you compare the details of the Engraving on the patchbox, say the small details in the engraving on the finale flower it is very similar to Shroyers hand shown on the KRA disc # 236 and 227, as is the detailing of the scallop on the side plates, and the three piece hinge on the door.
As I remember, the books show at least one Shroyer with this tang caving, instead of his usual flur. But I don't know that I've ever seen a Shroyer with a faux painted stock.
The disc only shows one rifle by Koons, # 151, and the engraving style is completely different in detail.
Just going by the afore mentioned, I'd say Shroyer made the patchbox, if not the entire gun.
So that's my 2 bits, if only worth 2 cents.
John