King,
I have seen an original 1866 Rifle with a shotgun butt stock and the steel Winchester butt plate.
It also had some 1873 features on it like the steel fore end cap and '73 rifle rear and front sights.
I liked it so much, I rebuilt a repro '66 Carbine in that style. It became my favorite main Match Rifle.
I have never had anybody question me regarding its authenticity.
My best,
Blair