The USSR at the time was producing both heavy tanks (IS series) and mediums (T-44, 54, 55 etc), prior to going for the dual MBT line which they found could carry just as powerful a main gun but 20 tons lighter (cheap and cheerful T-55, T62 line or expensive composite armour, gas turbine, autoloader line as in the T-64).
The heavies were based on the IS-2 line, then IS-3 a couple of hundred IS-4, the IS-5, IS-6 and IS-7 were various protypes only, the only other which went into production was IS-8, which Kruschev renamed the T-10 after de-Stalinisation of the USSR.