Looks like (A) a Joseph Needham patent side-lever fastener (rotating bolt single bite snap action, self half-opening), patent 1544 of 1862, or (B) a J. Smith of Birmingham side-lever, patent no. 3171 of 1863. However, from the shape of the bite, I'm pretty confident it is the Needham action. There should be a patent use number somewhere on the gun.