They are from a fairly early hammer gun, probably Grade 0 as they are marked Plain Twist. That was the lowest grade. If there is a serial number on one side of the barrel lump, someone will be able to tell you more.

I would double check that the barrels are 12ga. The 0 stamped on the rear lump may indicate the frame size. 0 frame guns would normally be 16 or 20 gauge guns. BUT, those barrels are early enough that they may pre-date the consistent frame size stamps.

----MattH
President, Ga. Vintagers