IMO both bases are bench-made, perhaps altered from another style. The key in my mind is the barrel-shaped tunnel addition where the SG swivel rod goes through. The rear base appears to be identical to some others on prewar rifles having non-detachable swivels, with the original tunnel cut away and the new SG-size tunnel then being attached, probably via silver-solder.
The front base's tunnel appears to also be an addition although the base itself is not familiar to me like the rear base.
However it appears that these SG swivels were made and sold by folks other than Winchester and so these could be bases that were supplied by the swivel makers themselves.
Regards, Joe
