In spring 1949 the OKB IK SV (Special Design Bureau of Engineering Committee of Ground Forces) under the command of A.V.Kravtsev started to design a family of fighting vechicles, the K-91. Finished projects included two heavy tanks (K-91 variants I and II) and a tank destroyer (K-91 variant III).
K-91 was meant to replace the existing tanks, combining the features of medium and heavy tanks, in other words, being the first Soviet MBT, though the concept of MBT itself wasn't developed yet.
K-91 I was designed in way not really traditional for Soviet tank design school. It had the engine in rear part of hull and transmission in forward. Drive shaft was placed under the turret basket. All the crew of four was placed in turret, including the driver, who was sitting on a rotating platform and this way was always facing forward. This all and the use of flat-12 V-64 engine (the predecessor to 5TDF) allowed to keep the height of K-91 well below 2300 mm while keeping it armored at least not worse than existing heavy tanks of that time.
Analysis showed though that these designs don't have any real advantages over existing tanks and project was closed in December 1949.
Maybe this engine really existed, maybe it didn't. I didn't find enough data on that besides some references.
A 100-mm rifled gun, designed to replace BS-3 field gun (as D-46) and D-10 tank gun (as D-46T). In the end D-46 failed to noticeably outperform the existing weapon systems and this project was cancelled as well.
(PS: I really need to work on making environments)