Most literature refers to the IS-7 as one tank, but this is far from the truth. Like many tanks, the IS-7 underwent a long and complicated development path, which started out with something completely alien to anyone familiar with the end result: Object 257.
The first thing you might notice is the suspension. Since there was not enough room for torsion bars in the hull, the designers used volute springs in a layout somewhat similar to the Sherman. The gun is not the 130 mm S-70 yet, it's only a 122 mm gun with a 1000 m/s muzzle velocity. The front armour of this beast was 150 mm at 60 degrees. The turret was almost impenetrable; the limit of damaging the rear surface (far from complete penetration) with its own gun was point blank.