Heres a Union Case made by S.Peck & Co made from Bois Ducri. Nope? me neither so I had to look it up.
Bois Durci – A hard, highly polishable composition, made of fine sawdust from hard wood (as rosewood) mixed with blood, and pressed.
Measures 2″ x 1 7/8″ x 5/8″
It hasn’t got a tintype or ambrotype inside but a paper image of some sort. Maybe this is how they were supplied from the manufacturer with a mock up or maybe someone chose to use a paper image rather than a plate?