Hi Paul,
Not absolutely sure, but I think it is Flensburg, in Schleswig-Holstein, Germany, currently just on the Danish border. In times past, part of the Danish Kingdom.
we have a terrible example of the Flensburg mark here ->
http://www.925-1000.com/Fgerman_marks_b1884_2.html" onclick="window.open(this.href);return false;
Regards, Tom