Gosh, you post some Big Pictures!

I believe your sling is commercial and of a type that was popular in Europe and Britain (including use on shotguns).

It should have two of the "sling-hooks" which are intended to attach to a simple "eye-fitting" that screws into the butt and forearm, or an "eye" brazed/soldered onto the barrel(s).

IIRC - Winchester also offered this sling arrangement on a custom basis.

This system offers a quickly detachable sling, but, can be noisy with metal on metal contact.

The use of this "sling-hook" on your Krag is rather redundant, since you already have front and rear swivels.

The "sliding-buckle" is clever, secure, snag-proof and quiet. I don't believe it is "military". A smaller version is seen on some binocular cases.
A buckle of similar type may be found on some Horse Equipment.

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