Many model 1899 carbines were altered by civilian owners to satisfy personal tastes.

I suspect those two plugged holes on Mike's stock could be the remains of where a wood or metal pistol-grip was anchored.

A surplus metal grip for the model 1873 Springfield use to be readily available. One of these could have been screwed to Mike's stock and repositioned for a better 'feel'.  Such a device could have made depressions in the wood similar to those on the stock in question.

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