I'll share my observations of NE rifles... I have 11 of them (more or less)... none quite as nice as that one but I've never been in a position to afford the really fancy ones. I have been looking at them closely for better than 30 years. I think it is a mistake to apply the same criteria used to attribute unsigned Pennsylvania rifles to NE rifles. In the first place, virtually all NE rifles were made in Worcester County Mass and its nearby environs. As a result, it is very likely that every maker, and nearly every potential customer, was familiar with the work of the other makers.
It is also very likely (I am convinced of this) that the overwhelming majority were made for use in volunteer militia rifle companies and we can expect a degree of uniformity that would not be likely where the use was primarily civilian. The flint rifles are almost all about .54 caliber. I have one that is actually .69 (with rifling still in the bore). I've never seen a flint NE rifle smaller than .50 caliber... so, they exhibit a degree of uniformity that is quite unlike anything similar anywhere else in the country.
Then we have the import problem. All of the locks are imported, Birmingham products. Some have the names of Boston hardware or gun dealers on them... but that doesn't alter where they were made. The mounts also appear to fall into readily identifiable types... especially trigger guards where identical examples can be found on singed rifles by different makers. I suspect they were imported as well, or at least many of them were.
There is also a problem with proofs... we presume rifle makers made barrels but there is little or no evidence of this. From 1804 Massachusetts had a proof law that required that barrels "made in Massachusetts" be proofed. The problem is that, while nearly all muskets have the marks, virtually no rifles do. I don't have an answer for this but the implication is that the barrels weren't made there and, therefore the letter of the law exempted them from proof. I'm sure this wasn't the intent... but, for instance, Silas Allen Jr. was actually an appointed prover of firearms and his rifles never have the marks. The only rifled barrel I have ever seen with the proof marks is on a Ruggles underhammer and we know that the Ruggles brothers made their barrels themselves.
There is a serious, scholarly work in progress on this subject, although the emphasis is on the Boston Gun Trade. It will probably be another year or two before its ready for print but we have already come up with a large amount of previously unknown (or unappreciated) data on the subject.
Muskets were commonly advertised as being available in lots of 60... the minimum size of a volunteer militia company. Everyone had to provide their own rifle or musket but its likely that only the volunteer companies were organized enough to purchase them in lots. But... to provide 60 muskets, or rifles, in a timely fashion they either had to be in stock or readily available from outside source. (Which implies they were imported and we know this to be the case for muskets.) I don't think any of the makers had the wherewithall to produce 60 rifles in a month or two...
The Boston rifle volunteers were organized in 1814. The organizers appointed a committee "to find if sufficient rifles are available to arm the company," implying that none (or very few) of the members owned a rifle. As they were all successful merchants and seamen, and all exempt from militia service, this seems likely. Less than 48 hours later the committee reported that there were enough rifles available in the city... I'm not sure you could find 60 rifles in Boston today!
I suspect... and I freely admit I have yet to prove this although I think I am getting close, that many rifles, possibly semi-finished, were imported in batches. If so, this would go a long way to explaining their uniformity. Of course, some were made here. Northern RI maker Welcome Mathewson (who learned his trade in Sutton, Massachusetts) had a peculiar, archaic style that I suspect allows us to make wary attributions but... as an example of how fraught with difficulty this can be, I have a rifle with all the distinctive Mathewson features, except its signed "W. Allen – 1817". I've yet to identify W. Allen though the natural presumption is that he was a relation of Silas and Silas Jr. To make matters even more confusing, the "1817" in the inscription is on top of a filled in rear sight dovetail so the signature cannot have been on the barrel in its original state!
Joe Puleo