The German spelling for Cologne, varied over the years. So in 1900 the Prussian Minister for Interior Affairs, issued an edict that the correct spelling for Cologne was Cöln. However, on the 1st February 1919, the mayor of Cologne, Konrad Adenauer, stated the official name was again Köln.
Probably most people continued to use the spelling they preferred.
Purchased steins from: Villeroy & Boch - Mettlach.
Reference:
Decorator's / Vendor's rubber stamp on a Villeroy & Boch - Mettlach stein, mould #1526. 