'fourbore' - The picture you have 'marked' is an Italian walnut stock that appears to have been stained (and maybe varnished or shellacked) at one time.

This was done to some of the Italian stocks (locally, on a company level, I believe) to make them better 'match' the other Krags in the unit.

To me, it appears the added 'finish' has been stripped-off to restore the original appearance of the lighter Italian walnut. The dark area, you have outlined, shows some 'residual stain and sealant'. (I don't believe this wood to be 'oil soaked').

FWIW - I don't think Springfield Armory 'stained' the Italian walnut stocks. It is possible this was done locally by Military Units, after the 'accepted arm' left Springfield Armory.