Other than the superstructure changes, the Ausf F had a new conical design of the idler wheel. By May 1940, additional 20mm armor plates had been rivetted to the front of the turret, superstructure and hull front of approximately 70% of Panzer II Ausfs c, A, B and C. The remainder had been increased before the invasion of Russia. The armor on the early models is thinner than the Ausf F and in the same shape, but the rivets would be seen. The eight periscope commander's cupola was installed starting in October 1940. Prior to that, the squared, split-type commander's hatch, bullet splash guard and dummy commander's periscope were all features of models preceding the Ausf C. Tool storage, storage bins, jack placement and other small fittings on the fenders are also different. I'll have to check and see if I have any blue prints.