Complete parts gun. Not even the barrel and receiver are original to each other.
Barrel is 1916 dated.
The barrel is not from a cut down long rifle. It is from a stutzen or a carbine.
The 'correct' sling for this would be a Bulgarian sling. Leather or web, loop on one end and a hook on the other with a few holes in the sling for the hook to go through. Not all will fit all guns.