I see that no one has responded to your question yet, so I will take a stab at it. I had a Francotte with a Greener cross bolt, and the top lever had a flat wedge shaped projection (with the large end toward the front) on its front edge that went under the top of the standing breech just to the left of the rib extension, and fit into a longitudinal slot cut into the back of the cross bolt.
Pushing the top lever to the right forced the left edge of that projection against the left end of the slot in the bolt, pushing the bolt out the left side of the standing breech. When released the top lever return spring forced the right side of the projection against the right end of the slot, moving it back into the locked position. In my gun there was no separate spring or any other parts, the bolt depended on the top lever spring for tension.
I once saw an illustrated parts list for an older Bernadelli Gamecock gun (maybe in a 1960s vintage Shooter Bible?) that showed the top lever with its projection, that visual might help you understand how it works. Unfortunately, I no longer have that Francotte to take photos of.
In hand it seems very simple, but I suspect that the shape of that projection and the length of the slot are critical for it to work properly.
