The master key is the one without the little projection on the end so it can be inserted in situations like this (i.e. the key doesn't have to fit in via the slots on the outside of the barrel, it can just line up with the inner barrel tab). If you don't have a master key just carefully file off the small tab and you have instantly made yourself a masterkey.
I don't know how clearly I have explained this but it will definitely solve your problem.