The new 40, 45, and 50 hp CUT's use this same frame.
The lift capacities, sizes, and weights are similar(but not identical) for tractors in each class, and the transmissions are similar in the same class.
The classes are just specific to New Holland and Case/Farmall CUTs and SCUTs. It does not apply to all brands. The lines are beginning to blur even between NH models as they change model numbers and features so often. As Chris mentioned, the tractors within a particular class all share the same basic frame size (running gear - chassis) and weight differences of only a few hundred pounds. Currently, all new Class IIIs are 4-cylinder engines. In the past, the TC35 was a 3-cylinder. Now, Class IIIs all are in the 23xx model number range.
Confused? Well, join the crowd. Can you imagine the frustration of a dealer who has to keep up with three different model number ranges for the same sized tractors?