Dragonlord
Fantasy Warlord
Nashorn I believe, the only other vehicle I can think of the with 128mm gun was the Jagdtiger, which was not open topped. As I have no idea who the guy is that part is up to someone else.
Sorry, not the Nashorn either!
As a further hint: this was a prototype, only 2 were built, but those 2 were used in battle at Stalingrad and were quite successful.