Thanks AG2012, that's rather interesting and I suppose it could be fake as I think you are suggesting. However, do they bother to fake brooches such as these when they could steal more money by faking 'higher value' objects? I wondered for a while whether someone may have overstamped the original mark, but the other side is enamel, so either they would have damaged the enamel or gone to a great deal more effort to remove and replace it. Looking at the back, it doesn't look as though the enamel section was removed. Another thought is that perhaps the pin is a later replacement. I haven't asked the owner whether he thinks that may have happened. The brooch is in America and I'm in the UK, so I can't check myself, so I need to ask.
Note as I said earlier, this is not my brooch so I have no interest in trying to prove it is real or fake for that matter, rather I'm much more interested in the discussion.