Very, very well said, Ken. My feelings exactly. I use the lunar landing negatives as my example but it is exactly the same.
No, the reason they are breathtaking is that while looking at them we suddenly come to realize that those fragile plates in our hands were, at the moment those four souls finally hung, physically present inside of Mr. Gardner's wet-plate camera, situated on the second floor of that building overlooking the gallows, only about twenty yards from the doomed prisoners, on that long ago March afternoon. And now we are holding those exact same glass plates in our hands 148 years later thus affirming, via these singular first generation objects, that those events did, in fact, really take place.