So I went back and further messed around with the Mass Production Mammoth model, as well as came up with a breif description.
While the X-66 Mammoth Tank was a wildly successful battletank in the First Tiberium War, the costs involved in producing and maintaining such a gargantuan beast of a war machine kept the GDI from being able to field it in large numbers. The R&D Team behind the X-66 proposed a stripped down version, the M-66a Mammoth Tank (Mass-Production Type). The M-66a is roughly 3/4 the size of its predecessor and has only one 120mm cannon. In place of the other cannon is a mounted .50 caliber machine gun to protect the tank against infantry. The M-66a keeps the Mammoth Tusk missiles as well, although the launchers have now been angled back 15 degrees in order to intercept aircraft more effectively. With this new configuration, the M-66a could be fielded in battle at half the cost of the original Mammoth Tank, as well as being more maneuverable while only sacrificing a minor amount of armor protection. However the First Tiberium War ended before the tanks could be deployed to the front, and as such the M-66a became a relic, quickly handed down to second-line units and Special Forces teams around the globe. Many of these mass-production models were lost to time, some of them found their way into the hands of the Forgotten, and others were used by police forces in riots that occured as large portions of the Earth were evacuated as Tiberium’s hold increased on the planet.