Clamshell is a description of the added metal to the exterior side of the action body just where it comes into contact with the barrels. On some (mainly German) guns, these are in the shape of a clamshell, hence, the name. This one, like the Searcy, is a skinny clamshell (lets call it a razor clam). The added metal strengthenes that area.

Side clips: In Brit terminology, these are attached at the "canons", or "detonators". Aussies often call these "boobs" for obvious reasons. These are the rounded areas on the upper frame which face into the rearwardmost part of the barrels. The butt part of the shell case presses against them. Am I getting too personal here? I hear the Aussies are shy. In any event, instead of being flat at this point, some guns have "clips" which fit into the sides of the barrels. Theoretically, this provides a bit of strengthening reinforcement for the eccentric loads of a SxS. In truth, they are merely for cutting the fingers of the serfs who quickly load our brace of doubles. Musn't let them think they are too adept at these things.

As for Anson and Dealy, it is the basic locking/design system of most doubles since about 1880 or thereabouts. This excepts the French who are too weird to copy (vid. Darne system). Repeat after me, "The French copy no one, and no one copies the French."