The extra sight blade is probably for a Brenneke slug in the right shotgun barrel. This is somewhat common to find in guns that were not originaly regulated to shoot slugs to the point of aim of the rifle barrel and was added. The inserts were regulated by the adjustment wedges on the sides of the main housing that fits into the chamber, and the small rolled pin at the side fits into a small slot filed into the extractor to return them to the regulation position for tightening.

I have seen several S&Ks like yours, but none nicer.

Keep in touch; drillings are my thing.

L2S