Most Krag carbine stocks will have three-rod holes in the butt-trap. (This was so the machining operations were the same for rifle and carbine stocks).
Some early thin 'long-wrist' model 1896 stocks only had two holes. These are very rare gun stocks.
Supposedly, the carbine rod consisted of two sections and the sections were slightly heavier, than the three rifle rod-sections.
The easiest way to identify a U.S. Krag carbine stock, versus a 'cut-down' rifle stock, is by looking at the forearm-tip area.
The carbine stock will have a solid tip and the barrel-band will be retained by a flat band-spring, on the right-side of the stock-tip.
A shortened rifle stock will have a filler-plug fitted into the stock tip. (This hides a stock lightning-channel, that was exposed by the cutting of the rifle forearm).
The rifle barrel-band is clamped with a sling-swivel screw and retained by a cross-pin, through the stock forearm.
BTW - Krag receivers marked 'model 1899' were built as carbines.