This is the E-75, the tank that would have replaced the TigerII had the war progressed beyond 1945. Part of a rationalisation program which was to see many parts interchangeable between different tank models to help speed up and simplify production. The plan was to have an E-10,E-25,E-50(panther replacement),E-75 and E-100. The numbers refer to the weight of the vehicles. All of these only existed on paper and were never built although the hulls and turrets of some of these were constructed( the E-100 shared the same turret as the Maus but mounted a different gun).
The E-75 was to mount a 100calibre version of the 88mm and have infra-red equipement as standard. The bulge on the turret sides is the housing for the rangefinder.