FWIW - I have a photo of Danishsoldiers armed with Krags.
Their slings appear to look similar U.S. Civil War Slings.
The Slings appear to fold back on themselves, through the upper swivel, and adjust with a central metal hook or device, that 'hooks' into positioned holes in the center of the sling.
The other end of the sling folds through the 'guard-swivel' and buttons over a stud.
(A repro. C.W. sling, with one hole for 'taught' sling and one hole for 'carry', would look good).