:haha: Good question - that hadn't occurred to me!
Looking at the photo (enlarged) the patchbox cover looks to be of different wood than the stock? (I just discovered that the original was lost, and it was replaced years later by the Smithsonian.)
The patch box may have served the purpose it does on a longrifle. Or, perhaps it held a tool of some kind - or a small bottle of strong rum to fortify the man who had to fire such a gun. :grin:
According to the USACMH, it was made at Rappahannock Forge at the direction of the Virginia government, about 1777. But, other than commenting that the gun has a sliding wooden patch box cover, the article makes no mention of its purpose.
Convention? (These guns show influence from the early Germanic style). (And such a cover on a fixed mount gun did not present the problem of loss that it would on a smaller, portable rifle.) No corrosion problem from salt air exposure (as opposed to brass or iron)?
I have been searching for an answer to your question, but I'm just guessing. You folks are the ones more knowledgeable than I ...I suspect that I will be enlightened shortly.