The original post generated a lot of "interest" but there are a few points that need to be cleared up.
Apparently, some think that it was my intent to compare American doubles to English (or Italian, German, etc.) best guns. It is not. It was, and is, meant for the edification of those who would say nonsensical things like "Classic American doubles are the equal of anything from anywhere" or the equally ignorant, "There's no such thing as a best gun, it's just British pretense". Believe me, there are plenty of American shooters that believe this way about their treasured doubles.
I evaluated each of the guns listed based upon their own merits, or lack thereof. Anyone with a modicum of mechanical comprehension would not compare these guns to bests from anywhere, since American doubles were factory mass-produced, consumer items, like toasters. The highest grades of American doubles were invariably mechanically and materially the same gun as the lower grades. Pretty engraving and wood does not a best gun make. I've said it before and it bears repeating, a best is a best even before it's engraved or finished, and no classic-era American double fits that description.
What I wrote was based upon observation and experience, not uninformed opinion: in other words, factual truth. If any are offended by that (and there have been a few), that was not my intent but it's also not my concern. There are any number of collector-types that have apparently sworn a blood oath to their chosen favorite brand, and part of my intention was to provide some actual knowledge about the workings of the guns that some of these people seem to lack.
As an aside, it's a bit surprising how many collectors, many of whom possess encyclopedic knowledge of their favorite brand's company minutiae, are utterly lacking in the ability to vet quality workmanship or the mechanical condition of the very guns that they collect. The fact is, one is buying the gun, not the company's history and when evaluating a potential purchase, all of that knowledge of company "history" is exactly worthless. Enough said on that subject.
I've also been asked why some makers were omitted, names like Baker and even Crescent. That's because those particular guns were barely a step away from a gas-pipe strapped to a two-by-four in build quality and are not worth mentioning any further. A couple that I did neglect to mention are the Remington Model 1900 and the Syracuse Lefevers. I will correct that presently.
Remington Model 1894/1900:
This is basically an A&D boxlock and because of that, it is a robust, reliable design. Perhaps the A&D patent had expired by the time the Remington was built and that is why they could build the gun without taking out a license as H&R had to do. Perhaps they actually did take out a license.
I do not know the answer to that as I'm not a Remington historian and honestly don't care. That particular bit of knowledge is of no value to me as far as the gun on the bench is concerned. As would be expected from this company, the workmanship is of uniformly good quality for a production gun.
Syracuse Lefever:
Early (pre-1900 I think) Lefevers were an odd hybrid of boxlock and sidelock, with the sears and their springs being mounted on the sideplates and the hammers and mainsprings mounted in the frame. The early guns were also possessed of "uncle Dan's" gimmicky adjustment mechanisms for just about every wear point in the gun. The actual value of these wear adjustments is debatable and they were omitted in the later guns. The later guns were also true boxlocks (sideplates notwithstanding) with the sears moved to the frame in an overhung fashion. The one adjustment mechanism that did make it to the later gun was the famous "ball and socket" barrel hinge. The adjustability is a byproduct of the fact that the ball itself simply screws axially into the frame knuckle.
Lefever collectors will wax eloquently about how all doubles should have been designed this way but the truth is that, much like Brown's rotary bolt, it's just another way of accomplishing a goal. Not better, not worse, just different. Where it was superior was in the production of the gun, when, instead of blacking the barrels down to the frame in the traditional time-consuming manner (which few American makers did right anyway), the barrels are assembled to the frame, the ball is screwed up tight and the job is done. The late Syracuse Lefevers really only have a few legitimate engineering issues. The first is that the hammers are pivoted in cantilever fashion from the frame, with no outboard support for the pivot. This is probably more academic than real since the hammer pivot pin (which also makes up half of the cocking mechanism) is of a fairly large diameter and derives sufficient stiffness from its diameter. Another is the geometry of the cocking lever. The Lefever's cocking lever is fairly short and engages the barrels a significant distance aft of the barrel's pivot point. This, combined with the fact that the cocking lever also rotates the hammer pivot pin itself in order to raise the hammers to full cock, makes for a gun that takes noticeable and considerable effort to open when both locks are tripped. In English, the Lefever's cocking mechanism enjoys less mechanical advantage than most other designs. Lastly, the design of the cocking mechanism demands that the hammers float on the pivot pin, while the cocking lugs on the pin are fairly small and engage in an arc segment cut into the hammer itself. This reduces the amount of contact in the pivot area but more importantly, during cocking large stresses are imposed on a very small area where the cocking lug of the pivot pin meets the notch in the hammer. I have seen the cocking lugs worn to the point that the hammers don't go fully into bent (full cock) when the gun is opened as far as it can go. I've also seen hammers crack, with the crack initiating at the square cornered opening above the pivot hole. Lefever's ejectors were as unique as the rest of the gun, with the mechanism being completely contained within the frame and powered by the mainsprings.
Some guns will be found with a screw that retains the extractor in the barrel. This is redundant as the only time the extractor can come out is when the barrels are opened and at that point, the cocking lever (which also cleverly acts as the extractor cam) retains the extractor in the barrels. I suspect that the retainer screw was used to prevent the extractor being lost when the barrels were dismounted from the frame.