SU-100Y
A Soviet prototype self-propelled gun, developed during World War II but never put into production.  It was a variant of the (also canceled) [[T-100]] heavy tank that had a 130mm naval gun in a fixed casement, and was intended to destroy bunkers.  Only one prototype was ever built, and the record is uncertain as to whether it was ever used in combat.

With its distinctive flat-armored, rectangular fighting compartment, the SU-100Y has acquired the affectionate nickname "Box Tank" among players of video games that feature it as a playable vehicle (a convention that started with players of the online multiplayer battle game [i]World of Tanks[/i], in which it is included as a tank destroyer despite the fact that that was not its actual function).

The real SU-100Y prototype still exists, in the collection of the Kubinka Tank Museum in Russia.
su-100y