Regarding Stützhakenverschluss; can I help partly answer this in a round-a-bout way:

My German cafe owner Helga was showing me a picture on the wall of her hometown, Wuppertal. It showed the unique suspended railway there. She couldn't think of the engineer's name for the braces or supports that hold it up. I happened to have a Schüler-Lexicon that named them as Stützen. Now this is, of course, also the name of the Stützen rifle which we call in English "fully-stocked". We learn from this, (I think), that Stützen means that the long stock is seen as a "support" and that it does not mean it's a carbine as it is so often translated as, (some Stützen stocked rifles are, of course, much longer than carbines. I have an example myself).

The German word Stütz means "brace" in English, hacken means hooks, and Verschluss simply means a lock. So I guess you're referring to the "bracing hooks" alongside of the barrels that lock it into the receiver and so reinforce the whole action. As to what it's called exactly in English, I cannot say. I have an old (1960) Gun Digest that has a full-length article on Wilhelm Brenneke and his designs. It includes illustrations of this very type of action. Annoyingly, it does not give a name for this bracing hook system. It seems to me that, the Browning (Miroku made) Citori and the current Beretta O&U line all use a variation of the original Brenneke Stützhakenverschluss design.

The shooting terms dictionary; WÖrterbuch der Waffentechnik, English-Deutsch, Deutsch-Englisch published by DWJ Verlags-GmbH, does not translate Stützhakenverschluss. However I hope this helps somewhat advance the discussion.