Oleblu, please check your U2U box. Before thousands of you jump my case, yes there was a 4 inch cylinder released mainly for the 770/880 Power traction hitch so yes there are 3 different sizes of hydralectric cylinders. Group one hydra-lectrics came out in 1950 same year as the diesel engine. Maybe we should put a tech type article about the difference in hydra-lectric units for the magazine