Depends on the year it was built. Look at the top side of the rear axle, the year of manuf was stamped on many of them.
The 9N where the earliest models made from 1939-1942
The 2N was built 1942-1947.
The 8N was made from 1948-1953
If its a 600 series,(1954-1957) you can determine year made...