I know it is not common, but I have seen Norwegian pieces of 900 fineness. There is an example of a Marius Hammer 900 mark here -> http://www.925-1000.com/Fnorway_marks.html
Perhaps the standard was used on some pieces meant for export?
I don't know if this brooch is Norwegian, but it certainly looks to be of Scandinavian or northern European origins. There is no registered Dane using PBH and the marks are atypical for Germany. Does anyone have an idea where it could be from?