The cast / rolled hull predated the 76mm turret. So late production 75mm M4s had the hybrid hull. For some reason, when the M4 hull was subsequently upgraded to the all-welded large hatch version, it was only used for the 105mm turret. The US Army had decided to standardise the M4A3 for its own use by then, so it seems strange they'd bother continuing production of the M4 just for the 105mm version. Maybe it was because there were never enough M4A3s.