The first three digits are the "kind code" designating what type of machine it is, 205 being a garden tractor. The next 4 digits are the "machine code", in this case 0590 designates it as a model 1100. The "U" designates it as made in the united states. Finally the numbers after the "U" are the actual serial number. Since the first number for march of 79 was 648387, your tractor 648587 is the 200th tractor from march of 79. That number includes ALL cub cadets made as there was no break in the numbers for each different model and they just gave sequential numbers to the various models in order as they came down the production line.