Well found KC. I did see that Liveauctioneer one, and that was one of the links I posted that somehow 'broke'? That one was also a pen rest and went for an incredible $1800!
However, that second one you found from the Civil War site is almost identical to mine ... haven't been able as yet to blow them up to compare the script, but they look like two peas from a pod. It is possible that this was a common sort of trench art during that period. And what a difference ... $18000 and $250!
The base, I am in agreement, probably was a later addition, and it's not as cool as marble should be, I don't think it's soapstone (pretty sure it's not) and I think you're right with travertine.
Later on ...
I've had a chance to compare them, now, KC and they are almost identical, except mine has the 'later' stand. I am beginning to think that these were possibly made as early tourist pieces? So not unique, but still possibly not all that common, and still interesting