I don't recall seeing this on a sight before:

The elevation 'slide' on 'OBXJER's Krag appears to have a small  'bushing' (on the left side). Maybe, its just tool or vise marks. It looks like the old CBS 'eye' logo!

The small hole, on the left side of the slide, guides a 'spring-loaded' pin with a flange. This well concealed 'pin with flange' is tensioned against the serrations and groove that runs along the left-side of the leaf. This allows the slide to move 'smooth and controlled' along the leaf when the 'locking-screw' is loosened.

(p.s. - I learn something new everyday. I went and looked at six model 1902 sights that I have, on guns and loose. None of them have the 'projecting bushing' of the OP's sight. However, at least three of the slides, although perfectly smooth in that area, have a faint change of shading in the 'browning'/blue. This suggests the slide hole was bored across the piece and then the left side received a 'bushing' of a slightly different steel. On my sight slides, the margins of the bushing are revealed by change in finish color. The OP's slide has the 'eye' shape because the round bushing was ground to the slide's radius).

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