I think it changes too frequently to get a good single answer. The modern format seems to be four digits. The first is the chassis size, the second the relative power, the last two the model lineup - 20 for the standard model but sometimes it's different (I haven't checked the complete list of reasons why). For example, my 2520 is the smallest CUT chassis and in between the 2320 and 2720 powerwise but about the same size as those two. The 3520 is larger and more powerful than the 2520, but it still sits between the 3320 and 3720. However there's also a 2305 and a 3203 just to keep things confusing. Just to add more confusion, in the 70's there was a large tractor called the 2520, so I need to be a little more explicit when I try to hunt down information. Generally I just Google the model number to see what it looks like and then compare HP to get a rough idea of size.
