A couple of points to add to this discussion...
Under a law passed in 1757, it required a license from the Privy Council to export military stores of any kind, including all guns and gun parts as well as gunpowder from Britain. This extended to British colonies and to Ireland as well. The council embargoed shipments to America in 1774 and the embargo was not partially lifted until December of 1792 and, even then only for swords, pistols and fowling pieces which had to have a wooden ramrod and not be fitted with a bayonet. One need only read the plaintive advertisements in American newspapers for anyone who might be able to make a gun lock (clock makers, locksmiths etc...) to understand that this was a skill that was almost completely lacking in America as late as the Revolution (and, I would argue, for a long time afterward).
Thus, the sale of obsolete British arms in America just prior to the Revolution is effectively impossible. While the odd example may have reached these shores by some circuitous route, the Marine & Militia pattern musket must have been virtually unknown. Most were issued to the British militia - another subject altogether and one that is poor understood today - but one that largely precludes their coming to America.
Hardwoods were the single largest export from America from colonial times through the 1820s or 30s. In keeping with Britain's mercantilist trade laws, practically all of it went to England until after the Revolution. Wood that was suitable for furniture was often suitable for gun making. Unfortunately, the trade records rarely specify what kind of wood it was but soft woods were readily available from Russia and the Baltic (a much shorter journey) so virtually all the American exports were hardwoods. I do have one period reference to the export of maple gun stock wood to Liverpool, albeit at a much later date – 1817 – but I would not assume that was the first time it was done. Except for a short period around 1800-1805 when we see some used by top end London makers, maple was not a popular stock wood in England. But, even though shipments had to go to England, there was no barrier to trans shipping goods from England to the continent. That was commonly done and constituted a considerable portion of British trade and as long as the goods traveled in British bottoms, was encouraged.
Quite a lot of records exist concerning this trade but none are readily available via the internet. We've only begun to scratch the surface of what is available. If I had to guess, I'd say that collectors have grossly underestimated the sophistication of 18th century trade.
As far as this rifle is concerned, I'm inclined to think it's a high quality American restock of an earlier rifle, perhaps with a part or two replaced...if the butt stock was broken off, a new butt plate would have been needed and from 1774 to 1793 I greatly doubt that any older, usable gun parts were discarded because they were old fashioned.
