The Sturmpanzer (Sd.Kfz.166), also known as Brummbaer or Sturmpz.43, mounted a heavy infantry gun in a heavily armored superstructure on top of a Pz.Kpfw.IV chassis; both newly built models and those that had returned from the field for repair were used. The superstructure was designed by ALKETT, and the weapon was a Skoda modification of the 15cm sIG 33. The weight of the modification led to breakdown of the suspension components, and the rubber-tired road wheels on first the front, and later the front two, bogies were replaced with steel-tired wheels. Early vehicles featured the fahrersehklappe 80 driver's visor from the Tiger I tank, but this was later replaced with periscopes for the driver. The M.G.34 ball mount was added after production had begun, and with this a commander's vision cupola was fitted. Early vehicles made from rebuilt tanks possessed a 5cm armor base, and therefore another 5cm plate was bolted to the hull front. Only one loader was found in early vehicles, and the crew in these vehicles was only 4 men. The StuH 43/1,introduced in December 1943, was a lighter version of the StuH 43 and featured a longer armored cover on the howitzer barrel.