RMC,
I can help you with the superposed question. Simply that one barrel is positioned above the other as opposed to barrels beside each other, side by side or juxtiposed(sic?).

Inertia triggers, certainly adequate. Nothing to chirp over. Some of the early Browning superposed had a double/single feature. The gun could be fired as a tradtional double trigger or a single trigger.

Someone will be along and will know a lot more than I.

Chief