I am not an expert - I am an amateur:

A model 1899 carbine, (#235XXX), built in 1899, likely has had three to four different rear-sights on it, during the course of its service:

1. When first built, it probably had the 1896 carbine rear-sight.

2. This could have been upgraded in early 1900 to the 1898 carbine sights developed for the 'hotter' 2,200 fps round. (This ammo was withdrawn from service in March, 1900, due to cracked bolts. Carbines in storage were not updated immediately).

3. 1896, 1901, and 1902 carbine rear-sights are all believed to have seen use, on updated/re-sighted carbines, when the 1898 sight and 2,200 fps ammo were abandoned.

4. For a time, the 1902 carbine rear-sight, (1,000 made), was preferred on Regular Army carbines, because it was most like the sight on the new 'Rod Bayonet' 1903 Springfield.

5. With 1905 changes to the 1903 Springfield, the 1901 Krag carbine rear-sight became preferred for Regular Army Service. It was most like the sight on the re-designed 1903. (1902 sights supposedly went to National Guard use and 'School Rifles').

The 1899 carbine, you have, is conceivably 'correct' with any of the four Krag carbine rear-sights (with appropriate hand-guard). The cheapest and easiest carbine rear-sight to find is the 1901 type.

To anticipate your next question: The 1899 'humped' hand-guard, for use with the 1896 carbine sight, appears in 1900.

The 1902 'humped' carbine hand-guard, for use with the 1901 carbine sight, was ordered in February, 1902, as a replacement on carbines (with the 1901 carbine rear-sight and regular hand-guard).

Potentially, lots of things can be correct on an early Model 1899 Krag Carbine - IMHO!