It’s that plastic nob, but they do have a habit of snapping if they are old.I’m guess it’s not clearing or you wouldn’t be looking at the water level. Sometimes they stick and don’t fully fill no matter how the valve is adjusted, squirting a bit of limescale remover about may work.
Does this look like the cistern water level is too low? Should it be at the (what I think is a) 6L line?
How would I adjust this so it fills fully?