Thanks everybody. Here's a bit more information...
As mentioned, three other guns signed by Cookson, of similar quality, exist. These are multi shot repeaters based on the Lorenzoni system. Here is a link to one that is in the Victoria and Albert Museum:
http://collections.vam.ac.uk/item/O77720/flint-lock-magazine-cookson-john/Another of these guns resides in the Milwaukee Museum. The final one was once part of the Locke collection in Cincinnatti and sold at a Christies sale in 1993. This particular gun is supposed to have solid provenance of being here in the states prior to the Civil War. As I understand it, the fowling piece presented showed up at a small sale in the Boston area within the last few years.
In 1756, John Cookson advertised the following in the Boston Gazette:
"Made by John Cookson and to be sold at his house in Boston a handy gun of nine pound and a half weight having a place convenient to hold nine bullets and powder for nine charges and nine primings; the said gun will fire nine times distinctly, as quick or as slow as you please, with one turn of the handle of the said gun, it doth charge the gun with powder and bullet and doth both prime and shut the pan and cock the gun. All these motions are performed immediately at once, by one turn with the said handle. Note there is nothing put into the muzzle of the gun, as we charge other guns."
In 1722 another Boston gunmaker, named John Pim, showed a repeating gun that was reported to discharge eleven times after a single loading in the space of two minutes.
The NRA museum has a Lorenzoni system gun, by John Shaw who is also reported to have lived in Boston.
So, the point is that there is a tradition of these repeating guns that showed up in Boston, likely from Cookson. Whether the Cookson in Boston made the signed Lorenzoni system guns and the fowling piece presented isn't known. He could have been a relative of the supposed London gunmaker or they could be the same person.
Cookson arriving in Boston in 1700 comes from a 1919 source titled "New England Families Genealogical and Memorial"
So, specific to this fowling piece, all this background information helps support the idea that it was here in this country during the 18th century and available for the military retrofit during the Rev. war period.
From a broader perspective, it's interesting that so many guns by Cookson of first class English styling ended up in this country. Lots of questions....