Compare this:
http://gundam.wikia.com/wiki/RGM-79_GM
To this:
http://gundam.wikia.com/wiki/RX-75_G...roduction_Type
They are both MP units based on first generation MS of the Federation. But look at them, and you can tell the MP Guntank LOOKS more advanced than the GM. Code Geass had cut corners with the Knightmare designs, because in many ways they are not important enough in the TV series to be worth it. But with the OVA, they can take their time.