Well, underlever, you were right: there are some "Kerstens" without the two inferior lugs... I don´t know how strong this type of system is. In the photos you can see that the opening lever ends in the upper part of the action. In my Sodia, the lever is longer, works in the usual way, and interacts with the lugs under the barrels.
This type of action is seen quite often, for instance inthe Holt´s auctions. All the doubles with Kersten lock <i have see are of the "four locks" type.

Herrdoktor