The serial numbers skipped around. The serial numbers were issued each week according to production scheduling. For example, they might reserve 80 numbers for 440's and the next 300 numbers for 1800's, etc. The next week they would start with the next set of numbers. With the serial numbers mixes in among different model tractors, it is hard to figure out how many of each were built.
When I get time I will try to look up the last number.