FWIW - attachments show - model 1898 rifle sight (note absence of serrations on left edge of leaf).

Also, sketches of Krag 1898 base and Krag 1902 base.

Bob: IMHO you have a sight that someone has put together from parts. This would not have been done by a U.S. Armory or Arsenal.

Likely it was done by a private dealer or gun hobbyist.

Your base & spring, leaf, 'locking-knob' and slide are all for the model 1902 sight.
Just the 3-notch eye-piece is for the model 1898 sight.

You have a non-official "mixmaster" sight, that only someone with surplus parts would assemble.

Local Boy- You are not wrong, but ...

(The Short Answer): The '1898 Slide' does not have a small hole on its left edge. The '1902 Slide' has a small hole on the left edge.

(Longer Answer):
Hidden from view, there is a 'spring-loaded plunger', inside the 'Slide', of 1898 and 1902 sights.
The plunger is on the left side, opposite the 'Slide-Screw' (which is on the right side).
When the Slide-Screw is "loose", the plunger maintains a bit of friction on the left side of the leaf, to keep the slide lightly in place. Once the Slide is adjusted for elevation, the Side-Screw is tightened to lock things in place.

On model 1898 sights the Leaf is smooth sided. So, the 'head' of the spring-loaded plunger is flat, (like a tack head). The plunger body, that fits inside the coiled-spring is short. These tiny parts fit into a 'blind' hole on the inside of the Slide.

On model 1902 sights the left side of the Leaf has serrations. The 'head' of the spring-loaded plunger has a raised 'dome' at its center. This  dome fits into a shallow groove that runs the length of the serrations. The plunger body that fits inside the coiled-spring is long. The plunger requires an outside hole to allow movement, provide guidance and vent debris, old grease, and dirt.
This "hole" is actually through a separate piece or bushing (apparently press fitted into the slide), called the "slide-shoe".
The inside surface of the Slide-Shoe has serrations, that lock into the serrations of the Leaf, when the slide-screw is tightened.

Aren't you glad you asked?

Actually, I'm glad I asked Chuck!

Thank you for taking the time to share your knowledge.  I've learned a lot.

I also appreciated you providing visual aids as I'm a visual learner myself.

BTW: Excellent pics rap1610!

p.s. Here's a mixed bag from my stash that I didn't realize I even had...guess I loose SIGHT of things from time to time.

Here is a real M1898C sight.