'unclefrank' - You have a complete model 1898 rifle rear-sight. It was designed by Lt. D.C. Dickson and is sometimes called the "Dickson sight".

This sight (model 1898) was calibrated for a higher velocity .30-40 cartridge, (220 grain RN projectile moving at 2,200 fps), that was decided upon in October, 1898.

Subsequently, it was concluded that this 'hotter' load was resulting in some 'cracked' bolts. (Cracks were appearing behind the single locking lug).

In March, 1900, the 'hot load' was withdrawn from service and velocity returned to 2,000 fps. The Dickson sights were eventually replaced.

Franklin Mallory wrote, in "The Krag Rifle Story", that the Dickson sight was used on model 1898 rifles assembled from August, 1899, to March, 1900.

The sight leaf on many Dickson 1898 sights was removed, put on a model 1902 base, and altered to make a variant of the model 1902 sight, with one notch and corners ground 'round'. (The model 1898 rear-sight bases became surplus). Approximately, 110,000 model 1898 leaves were converted, per Mallory.

Also, according to Mallory, the peculiar outside notches on the 'Dickson' sight were to compensate for 20 mph winds at 1,800 yards.

Attached photo shows a model 1902 rear-sight that was likely the type on your 1903 vintage Krag, when first assembled - IMHO.

How did an 1898 rifle sight get put on your 1903 Krag? (I don't rightly know).

It could have been  a hobbyist, dealer, WW1 stopgap or NG armorer, to replace a broken sight. 

---