Here 3 more photos. By the look of them, it looks like my rifle should have the slide lock: but, as I said before, I can pump it back and forth irrespective of the hammer position. Maybe the slide lock isn't working anymore because the slide and ramp are worn out (?). However I took the rifle apart and tried what you suggested: so when I push the pump forward, the magazine stop goes a bit down, but not enough (it doesn't go below the bottom of the magazine tube). If I pull the trigger, the magazine stop goes down another bit, but never completely below the bottom of the magazine tube.