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Since this hallmark (i.e. the combination of the date letter B and the William IV duty mark) was only in use for a month then the simple answer is that it is 12 times as rare as the alternative (or 1/12 as common). However, although I have seen this combination before I have to say that I have never seen the alternative, date letter B and Victoria duty mark, so it is possible that the earlier combination actually stayed in use longer than is claimed.