Regarding the Pacific K-1 sight - The base, normally, can be easily pulled to the rear and 'off' the receiver, once the 'lock-screw' has been removed.

Perhaps, corrosion, crud, and maybe paint, is causing the shaft to be hung-up or stuck in the magazine cut-off hole.

Soaking the shaft, (which is only 'press-fit' into the front of the Pacific sight-base), with some penetrating oil or 'Liquid Wrench' may dissolve the mischief.

Wiggling the base or tapping it "Forward" may free it up. Be patient and let the penetrating oil 'work'.

Caution - Avoid hammering on the front of the base, to try and drive it rearward.
The base and shaft can separate, leaving the shaft remaining stuck in the cut-off hole.
(You want the base and shaft to remain connected).

IMHO - At some point, someone painted your Krag's metal with Black Enamel Paint. Most of the paint has been removed.
The residue (which is still visible) can be removed with paint-remover or nail polish-remover and #0000 steel wool. Saturate small pads of #0000 steel wool with 'remover' and lightly work paint residue. Do this carefully and leave underlying patina.
Start with a piece like the Butt-Plate or Side-Plate.

Your metal should look a lot better with patient cleaning.

Attached photos show details of Pacific K-1 sight:

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