Howdy Lord Eoin,
I gather that you are not intending to use a sling instead of a gun cart foe CAS. If you do intend to use it for CAS, I would suggest you consider a gun cart. It not only provides you with a way to haul around ammo, tools, cigars, fired shells, etc. but all is a handy gun rack where none is provided at a given stage. I'm afraid I can't help with period correct swivels. Also having a sling waving around under your rifle while you are moving from target to target might be counter productive.
Reverend Chase