It is not Hanau - there is a misstake in
http://www.925-1000.com/Fgerman_marks_b1884_3.html
it is Bielefeld ca. 1750
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Makersmark (IG/G) = Johann Georg Gäntzer, *1712 Master in Lemgo, later in Bielefeld ca1745-ca.1760 - ca 1765 in Lemgo.
+1778.
Ref: Wolfgang Scheffler, Goldschmiede Rheinland Westfalens, p.73 no12 #139