A tractor built in October of 1955 would have been considered a '1956' model. The new production year usually starts around September. Tractors, just like automobiles, are manufactured on a model or production year. Tractor serial number, and not one of these other numbers. Various components may have parts or casting numbers. The engine, front axle, and cab may all have their own serial numbers.
There are usually a lot of different numbers on a tractor.
