Here are some photos I took of the Clarktor-6 towing tractor examples on display and in action at at the NMoUSAF, Arlington, Alabama, Castle AFB and Georgia.
More recently, The Clark Equipment Co. produces Clark Airfield Tugs
There are some examples that look similar to Clarktor tugs, but the fender differences are too great to overlook. I have placed them on the Unidentified Aircraft Tugs page.