I am thinking of doing something on the NE makers eventually but it will have to wait until I finish my current work on the Ketlands. In fact, I'm in England right now and going to the B'ham arms fair on Sunday.
I admit that, thus far, I've struck out in finding slam-dunk documentary proof of the importation of rifles by NE makers. I had hoped to be able to search the incoming foreign vessel manifests in the National Archives but it turns out that the Boston Customs House burned down in 1848 so the records of the most likely port of entry are completely lost. There are other Massachusetts records and I've just started on them but unless someone used a relatively minor port its unlikely I will find anything. However, I already have the proof of English-made rifles being exported to Pennsylvania (despite its large number of local makers) so it isn't much of stretch
As to proof... British law didn't actually require that barrels be proofed if they were intended for export. They usually were but I suspect that was because the buyers expected the guns to be British made and saw the proof as an obvious sign of quality. I think that may have been much less the case with rifles although I'd be thrilled to find a NE flint rifle with British proofs, I'm not hopeful. For those of you that have George Moller's "Massachusetts Military Shoulder Arms", most of the rifles and Mass. militia muskets are/were mine. I did the layout and design of that book and collected some of the illustrations.
As an appendix in the Moller book you will find a transcription of the orderly book of a Massachusetts volunteer rifle company. In a nutshell... the company was organized by wealthy merchants and ship owners at the Exchange Coffee House (the Boston version of LLoyd's Coffee House in London).
At the first meeting a committee was appointed to discover if "sufficient rifles were available in the city to arm the company." A minimum of 62 rifles were required - the minimum number for a volunteer infantry company. Three days later (if I remember correctly) the committee reported that there were, indeed, a "sufficient number of rifles available." That is a lot of rifles for a seaport city in a state where there was practically no "big game" hunting. It certainly suggests something other than a handfull of artist/craftsmen turning out rifles one or two at a time for individual customers. Others (notably Frank Klay) have suggested that most NE rifles were made for militia companies - not for hunting. Not every company had a matched set but rifles that are obviously part of a set are well known. I've had them marked "No. 6" and "62" and I've seen plenty of others that were numbered. Its really hard for me to imagine a shop - even a big shop like S. Allen's, with several apprentices, taking an order for 62 rifles, making all the parts and delivering them in a timely fashion.
A few other observations... generally the quality of NE rifles is higher that Pennsylvania rifles or, perhaps more appropriately, NE rifles are of a quality analogous to the best Pennsylvania rifles.
I have the Allen dates at home but won't be back until mid March. As an example of how confusing this can be, the usual published dates for the Pratt brothers, Alvin and Henry, usually reflect only the last part of their careers - both were actually born in the 18th century and could easily be War of 1812 makers.