Armbar, Beautiful rifle! It is a screw grip action as 500 Nitro says.
In your pic of the breech face you can see the 'screw grip' behind the
dolls head recess. When you push the top lever over you can see the slot
that the extension at the back of the dolls head slides into. When you
release the top lever,the 'screw grip' rotates over the extension locking
the barrels in the down position.