Jay, very good points. But even if it turns out it's a later image that was affixed to a used Williamson mount, given that Paul Messier feels almost certain it's 19th century, it seems very unlikely that it would be a late 19th century 2nd generation image - that would mean that someone had access to the original image for 25+ years, then made a copy, glued it to an 1865-75 cdv mount (correct period, by the correct photography studio), and then threw away the original image. Unless the original image was trashed and they were creating a replacement.
And if it really is a ca. 1870 image, but was made from a photo (as opposed to a negative), it's unlikely that anyone will be able to tell the difference due to the blurry vignette image and the fact that it would still be 140+ years old.
The other alternative is that it's a modern albumen that looks just like an 1870's one, but was made from an image that no one has ever seen before - probably the least likely scenario of all.
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