Might be the latter. WWI and earlier if a section of jaw was removed by bullet or tumor it was possible to "restructure" somewhat. Can't say what you found was for that, or even a cast to use for later making a more permanent fixture . . . but it's a possibility.
If you want to see horror Google: "WWI Jaw Reconstruction".
I don't believe it's a fossil - at least not a jaw fossil. Teeth & bone are different materials and the lower jaw is open.