thanks for sharing the info. I am afraid it is only partly true.
I inspected a bit my parts and consulted with Herr Reinecker, and I think I have the solution.
It is true that a sping plunger hold the trigger in a firing position.
First of all the recessed point you indicate is on the top when the trigger is locked in "armed" position (so it is useless), but there is no "recess" point needed on the trigger at all.
It would be sily solution, becouse it would also lock the movement of the trigger toward the firing button.
The engineers at Argus were very smart guys. They did it like this:
There is a ball pressed by a spring toward the trigger as you showed as well. But the ball is blocked from falling out only by the trigger itself.
They made a path for this ball around the shaft of the trigger.triggermech1
, on Flickr
You can see, that the ball runs freely around the trigger shaft but there is one slope with an edge to block it. When you "arm" the trigger you force the ball over this slope. The edge will hold it until you force it over back again. But you can move the trigger freely towards the fire button.
Clever, elegant and simple. German engineering:D