After the original RX-75 was introduced to combat, there were frequent complaints about the lack of pilot protection, lack of sensor capability, and lack of expanded ammunition capacity. In response, the ZODIAC Corp developed the RX-75D Production Type.
Moving the pilot cabin into the torso, adding a GM-style sensor suite, expanded communications package, and greatly improved ammunition storage, the Production Type addressed all the major complaints from the front lines. After some trial runs in combat, the initial wave of 75A series were recalled and replaced with the 75D.
Still not perfect, but I'm liking where this went after some input from Alex. Based on the RX-75D from 08th MS Team, more or less :-)