The only one that Ertl ever made was the 6600, but I have seen a 6600 that a friend of mine who collects toys had customized to look like a 4420. He had painted the cab black, and had put a header with the black reel from an Ertl 6620 on it. He also had decals for it as well. I still have a toy 6600 that I got as a present for my fourth birthday 25 years ago. The toy is in excellent condition, except that the yellow plastic cab roof has started to discolor slightly from age. The one I have has the lug rear tires to look like power rear wheel drive. If you can find one in excellent condition they usually go for upwards of $150, but I saw one on Ebay that was still in the original square "ice cream box" sell for $250 a couple of years ago. If any you have any of these old toys lying around, some of them are very valuable.