Prussia had had no imperial free cities. The East in general consolidated serfdom even as serfdom was weakening (de facto) in western Germany. When Prussia took over Rhineland & Westphalia it was likely a deceleration of ‘normal’ political development in western Germany
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But I don't think it's Sonderweg as long as I'm not arguing that there is a straight line from the late 19th century to the 1930s, and I'm not arguing that.