It looks to me that it's made of many pieces of MMF puzzled together. The truck itself look separate and hand painted, Each of the tires look separate. made a little thicker with treads imprinted. Then it looks like the "sprockets" were added last and painted with silver luster dust/alcohol. The dirt could just be globs of buttercream painted to make it look like dirt.
Sorry not too much help... Maybe you'll still hear from the original owner.
Good luck ~Mindy
Now that I look at the picture again, I think the dirt is MMF too. With the color marble through.