 Complete the following phrase:  While lighter tanks formed almost the entirety of the German forces, heavier tanks were at least at the prototype stage. In 1934, a number of heavy prototypes were constructed, based around either 75 or 105 mm main guns. Designated Neubaufahrzeug NbFz and very similar to contemporary Soviet and British designs, six were built by Rheinmetall and Krupp. Although they were mainly useful for propaganda purposes because only a handful of the tanks were made, three were actually used 