I am not an expert on the machines by any means. I am posting what I think to be true however I hope anyone will correct me where I am mistaken so I can learn more about these machines. Sometime in the early '50s the University of Utah built several prototypes of low ground pressure machines for Morton Thiokol for use in missile testing programs taking place on the salt flats. The most sucessful of these was the 4VL. It had a drive system where the the engine power went into a gear box where it was split into two output shafts each running through its own clutch and brake. Then two separate driveshafts took the power to two truck style differentials welded together with the outboard axles turning sprockets to drive the tracks. Pulling back on the steering lever first disengaged the clutch, then applied the brake when pulled further, turning the machine. This drive system was most likely very difficult to keep properly adjusted and at any rate not very strong. The last body style used had the reverse sloped windshield and would become the 601 series. The 601 drivetrain went with a Ford inline six cylinder engine, 4-speed standard transmission, and a OC-15 steering differential. The military spec machines used the Ford 240 cu.in. and the civilian version could be ordered with the Ford 300 cu.in. engines. Oliver Tractor Corp. made crawlers using Clark steering differentials, notably the OC-4 and OC-12 which were used in the Thiokol Spryte, or 1200 series, machines. The OC-4 was also used in the Thiokol Imp. They did not build an OC-15 crawler so I believe this differential was made for military contract. The military 601 was mostly used for airfield recovery. I am somewhat puzzled by the example I own since it has the 300 engine, no roof hatch over the passenger seat, yet was originally painted blue (Air Force). I have heard these vehicles refered to as "Trackmasters" and have a maintenance book cover that says that too. They are very heavy duty especially by snowcat standards.