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?