WIPAC CJ936 Stop button?

Can someone please clarify the purpose of the small brass(?) button below the front cover on the subject magneto please?  The parts diagram states that it is a 01080 Stop Unit, so I would think that the small 'tit' on the end should be depressed to short the mag and stop the engine.  Is this 'tit' meant to depress please, as mine doesn't.  I have given light persuasion with a rubber hammer but don't want to break it.

Thanks in advance. Oscar.