J. D. Dougall had a patent on his "Lockfast" action. (Patented in the mid 1860's - originally for a pinfire cartridge) On the standing breech face, there were two raised bosses/roundels that fit into a recess in the breech of each barrel. The gun had a lever along the right side of the receiver that, when turned down, moved the barrels forward approx. 1/8" and then allowed them to swing down.
Many others did something similar (perhaps upon expiration of earlier patents). Collath drillings have the same basic mechanism as Dougall's, but it uses a very large lever underneath the forearm/foreend to actuate it.
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