bsteele, I have thought more about your thought "The “stops” on the outside of the lock plate seem stationary. I don’t see any bright spots where they have moved if they act in an internal firing pin mechanism."
Maybe the stops are not stationary. Purdey might have fabricated them so they would not make marks on the sideplates when moving forward. Afterall, it is Purdey. The stops would not have to move very far to push the firing pin into the centerfire primer. Argo's illustration might be a similar design with the hammers striking a stop that is connected to a hidden firing pin behind the lockplate. Does this make sense ?
