Redesigned parts were most often phased into production after all the older parts had been used. New parts were generally designed to be compatible with tractors already in-use so older tractors could be easily upgraded if repairs were necessary. For this reason, buying correct parts for these tractors is not difficult, since there were basically only two models built from 1939 to 1952. It is so easy to upgrade and swap parts between tractors, few of the tractors still in use will be exactly as they were originally built.
At the time these tractors were designed, Ford Motor Company was using single letter designations for each new development project. Successful projects that made it to production such as the Model 'A' and Model 'T' are very well known. Many projects never made it to production. The letter 'N' was chosen for the project to design an affordable tractor using the Harry Ferguson hydraulic plow system.
The designation '9N' stands for 1939 Model Tractor. The 9N tractors were produced from 1939 to 1942. Ford Motor Company changed the model designation only when it was felt a new model was needed.
The Model 2N was a special case, created as a way to manufacture tractors when rubber tires, starters, and batteries were only available for vehicles directly related to fighting World War II. It is unclear why Ford didn't change the serial number prefix to 2N. Perhaps because the model designation was suggested by the Ferguson-Sherman Corporation rather than being a Ford factory model change. The new model may also have been a way to get around the Wartime price freeze.
All N-Series Tractor Serial Numbers were hand-stamped on the engine block, up high, on the left side above the starter. Look for a flat spot in the casting a few inches back from the oil filter, just below the cylinder head.
The serial numbers are usually difficult to read and they used some odd characters so a '6' will look like a lowercase 'b', a '1' looks like a capital 'I'. All 9N serial numbers will have a small star symbol ☆ stamped at the beginning and the end. Most 8N serial numbers will also have these star symbols. After serial number ☆ 8N433578 ☆ the stars were changed to diamonds (like this ♢ 8N433579 ♢ ) to designate the change to cast sleeves in the cylinders. Do not depend on the stars or diamonds when buying parts to rebuild an engine.
Many engines with the stars have been rebuilt using cast sleeves. Instead of the basic '9N' and '8N' model prefix, you may find some serial numbers like '9NAN', 'BNO', or '9NBN'. The 'NAN' designation was for a kerosene burning tractor. Mostly sold in Europe, they are very rare in the USA. A 'BNO 40' or 'BNO 25' serial number prefix indicates a Moto Tug. These were stubby tractors with smaller tires and special sheet metal body parts, mostly used at airports. The '9NBN' serial number prefix is for a military tractor.