Eventually, a butt-trap was adopted for U.S. Krag rifles and carbines. This provided a place to store Cleaning-Rod sections. Later, many stocks had the cavity altered to also store an oil container.

On later Krags, the top large hole, bored into the Stock-Butt, had 3 smaller holes (drilled into the bottom) that accepted 3 rod sections.

The early carbine stocks had just 2 holes, to accept 2 cleaning-rod sections.

(Mark - You have to shine a light down the top large butt-trap hole, to see the bottom and how the smaller holes are configured).

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