"Joe L." - Welcome back to the KCA Forum.

The "flip-up Peep", that you mention being present on your model 1898 Krag carbine, is found on most Krag model 1902 rear-sights.

The feature was also found on the first type of rear-sight that was used on the "Rod-Bayonet" model 1903 Springfield Rifle.

This sight is described in Mallory's, Brophy's, and Joe Poyer's books on the Krag.

Per Franklin Mallory, ("The Krag Rifle Story", 2nd. edition, page 130):

".... There are two basic models of the 1902 sight, the second differing from the first by addition of a sliding peep aperture. This device was commonly called a "Sergeant Peep", which was a misnomer because it was used by privates as well as sergeants. It was designed by Springfield Armory's veteran marksman, F. R. Bull. Few Model 1902 sights were issued without the peep plate".

IMHO - Although the 'peep plate' is quite common on Model 1902 Krag sights and the early (and discarded) rear-sight for the Model 1903 Springfield, I have never seen one on a 'Trap-Door' Springfield.
(My hunch is that it is likely some craftsman's 'innovation' and not official Springfield Armory work).

Attached photos: 1. 1902 'peep plate', 2. Graphic from early 1903 Springfield Manual. 

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