I have 2 of these in let properties, one is Open, and the other is Sealed. One has an AAV and a level switch and the other doesn't. I wasn't sure which was which so I had a look in the manual, and on the circuit diagram it shows "Water Level Switch" with a note against it which says "N.B. Not fitted on open vent models".
I also found this on Mike Bryant's
http://www.powermax-repairs.co.uk/ :-
3) Boiler refuses to light at all, showing the red lock-out light after three failed attempts to start. Several possible reasons for this, but the most common reason in my experience so far is a blocked automatic-air vent (sealed system versions only).
I guess it is easier to lose water and require a lockout on a sealed system and that is why it is on one and not the other.