As we know, the muzzleloading era has never ended. But 150 or so years ago, some poor, misguided souls began loading from the breech or wrong end of their guns, using little brass cans they called cartridges. These suppository guns are still used to this very day by the descendants of these folks. Hard to believe, but I've actually seen it with my own eyes!
Anyway, as production of these new-fangled gizmos ramped up, fewer proper, muzzleloading firearms were made. Due to the mass conversion to breech loaders, especially by the various militaries of the world, a surplus of parts for the early guns came to exist. Locks, barrels and furniture. The gunsmiths of Belgium used these parts to build trade muskets for the African trade well into the 20th century. Turner Kirkland, for one, used to have some of these parts in his catalog. So it is quite possible for a faker to find the necessary brass furniture to build any number of guns, or to copy an old part and use the new castings to build his fakes. Add a little patina, and suddenly the new gun looks old.
If old barrels are reused, the proof marks don't have much use in indicating the age of a gun. I have an original Enfield barrel on one of my pistols, and it has real proofs on it. The barrel is 150 years old, but the pistol is only 25 or 30 years old. The maker was an honest man, but with a little work, this gun could be turned into a reasonably good fake. The old adage "caveat emptor" is of profound meaning in the purchasing of old guns. It is very difficult to determine authenticity from a photograph. Some might say it's impossible.