Yes a black knob clips into the hole and you can use it to open or lock the door from the inside, my guess is that the original was either removed or been knocked out or something

If the lock doesn't work properly but you can hear it trying to work then i would say that there is something bent or broken inside the locking actuator which is stopping it from catching fully.
If no sound at all then possibly a wiring fault.
56 350 2.2 130ps mk7 with 2 side doors
65 290 2.2 125ps custom with 2 side doors
07 350 170ps
Ids v86 licence + v86.04 update + xprog
hereIds map
here