Yeah, a plug bayo would need a tapered butt to wedge into the gun barrel.
The tang nut is what is sometimes known as an ordinance screw; they are found (not exclusively) on military equipment, prominantly bayonet handles, for instance. It is turned with a special "ordinace driver" (not making up this probably obsolete terminology! ).
It leans me toward later 19th/ earlier 20th (in the private message I was picturing a threaded pommel with an emergent tang; that's the factor I think is perhaps more recent...) the blade looks older than that though, and perhaps the grip and upper ferule and pommel are replacements????
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