It has been suggested to the owner of this possible 'trial rifle', that a chamber cast be done. (It is pretty much the consensus of responses on this thread that the '.30-40 KRAG' marking was a later addition).

Some of the bolt 'internals' on this rifle have Belgian/FN ('Z') markings. (I suppose this could be a replacement 'striker-safety assembly' from a Belgian model 1889 Mauser).

I think this rifle is an amazing curiosity and it has come to be in good hands.

The Mauser Rifle Forum on Gunboards has some interesting posts and contributors. There are several threads that have Springfield Armory 'Mauser trial rifles' content.