The design was unusual because the main weapon – a larger caliber, medium-velocity 75 mm gun – was in an offset sponson mounted in the hull with limited traverse. The sponson mount was necessary because, at the time, American tank plants did not have the design experience necessary to make a gun turret capable of holding a 75 mm weapon.
https://en.wikipedia.org/wiki/M3_Lee
The design was unusual because the main weapon – a larger caliber, medium-velocity 75 mm gun – was in an offset sponson mounted in the hull with limited traverse. The sponson mount was necessary because, at the time, American tank plants did not have the design experience necessary to make a gun turret capable of holding a 75 mm weapon.