Model 1899 Krags were only built as carbines. During their service four carbine rear sights saw use: model 1898 (which was later removed from service), model 1896, model 1901, and model 1902.

It is possible some one removed an 1896 carbine rear-sight to sell it separately. (They can bring $500-600).

If a Krag was used for Hunting, the front-sight blade was often replaced with a commercial one.

Also, the carbine front-sight blade, that was used with the model 1896 carbine rear-sight, is shorter and its "c" mark is hidden, when the blade is in place. (My picture shows the higher "c" blade that was used with 1901 and 1902 carbine sights).

There was never a "C" marking put on Krag carbine bolts or butt-plates. (I don't know what gave you that idea)?