I think it was JTR who asked for help for new guys. Here's an important feature that, for me, kills a rifle as any type of representative study piece. Specifically, here we consider the marriage of an old barrel to entirely new wood and furniture. Again, I repeat, finding a bit of honest restoration around a lock, or even a foot or so of wood up front is to be expected, and if done correctly, a gun can still be studied as representative and this is good. But, when an old barrel is married to different wood, quite possibly old itself, then you have a distortion of historical authenticity to the point that the gun better be left alone, rather than studied, because no one can tell from the existing piece what things looked like when first made. Misinformation is more probable than "education."
This is one of the more blatant forms of subterfuge that we occasionally encounter, and I wholeheartedly concur with Henry's description of those that sell such specimens. Here is a dead give away. If there are additional holes in the wood where pins or wedges used to be, and or sets of tennon traces (unused dovetails or holes), welds or fill-ins on the bottom of the barrel, something is likely not right.
This is one of the things that may not surface in a standard photo and require physical examination. In that magical world of fairies, etc., I guess archivists could physically examine each item, then choose to photo and post the good ones. This being entirely impractical, we just need to know that such items are often entirely misleading.
Also, less critical, I do not personally like stretched barrels, though I have learned to accept them as part of the world we live in. When you are stretchin' you are guessin'. People will approximate based on distances between tennons, overall proportion, etc., but there are some assumptions, and choices are made among variables. Again, it is often impossible to discern in photos if this rather common procedure was done. Hope this helps.