Where Do 100+ Year NH Stamps Come From?

Full sheets/panes that were held back and then broken up years/decades later?

More than likely the vast majority come from the workbench of a re-gummer.

So the Scott prices include and reflect all those. But can you at least be more sure about them if you have a pair (or multiple) without any trace of cancel and an ok looking gum?

Well, technically a "majority" is more than half, so "the vast majority" is much more than half. But this statement is only even plausible for stamps much older than 100+ years ago. Regumming becomes "worth it" only when MNH stamps are scarce to begin with, and I'd think you have to get well back into the 19th century before you statement about "the vast majority" has any likelihood of being the case. By the dawn of the 20th century, stamps were being produced in such volumes where, for a variety of reasons, there were going to be plenty of unhinged stamps floating around, and at a time when there was no value to regumming those that were hinged.

That's the problem with generalizations. They are always true, until they are not.

As we learn about the areas we collect, certain characteristics begin to stand out. Some stamps tend to have brownish gum which is extremely difficult to reproduce, so if you see a stamp with whitish or white gum one should be suspicious. Gums also vary in texture- for example there were 4 printings of the Falkland Islands KGVI definitives and each has distinctive gum. The first printing (1937) almost always has "crackly" gum with crazes. The second printing (1939) has gum with vertical waves that is easy to spot once you've seen it. Gum on both these issues has some shininess. The third printing (1944) has matte gum that is whitish to slightly "creamy" while the last printing ('49) has very white gum.

A lot can be learned from common, cheap stamps as there is minimal motivation to regum them. I've noticed certain patterns in French Colonies stamps that help me make a more educated guess as to whether an old MNH stamp may actually be regummed.

P.S.

There is always a group of stamp dealers who will tell you that if you've built a certain collection with only MNH stamps, then you have a collection FULL of regummed stamps. They will also tell you they have rarely if ever seen a MNH in the area you collect, so it's not possible to find that many MNH stamps. I was told as much several times when discussing my KGVI collection with dealers.

These are dealers who assume their experience is everyone's experience and they know all there is to know. They generally are buying collections directly from collectors either at shows, through ads, or through auctions. Collector made collections prior to 1950 or so will almost certainly have a lot of hinged stamps, if not all. However as per other comments in this thread there are all kinds of sources of old stamps that don't come from collectors mounted collections.