In addition to having the usual tricycle front end configuration with TWO wheels, a "row crop" tractor can also be found with a ONE wheel front end. We'll use the Model B as an example. A late styled Model BN is shown here. The "N" stands for "narrow". The tractor ITSELF is the same, but collectors sometimes consider it to be a different model.
The following pictures shown the Model BW in it's unstyled and early styled form. This is known as the "Anteater Nose" for obvious reasons. The front pedestal is a completely different part than the one on the tricycle tractor. The width of the wheel tread is adjustable.

On the late styled Model BW, the front pedestal is "convertable". It can be changed to a narrow front without changing the entire pedestal.
Now... When the Model B appears with a "fixed width" wide front end, it is known as a Model BR. This is what is known as a "Regular" or "Standard Tread" tractor. In the case of the Model B, it really IS a different model than the tricycle. In some of the other later models the distinction is a little more hazy. See some more examples of "Standard" tractors, HERE. The Model B was also built as an "Orchard Tractor", an "Industrial" and a "Crawler".
The Model A, 60 and 620 also had modified versions of their "Standard" tread tractors that are called "Orchard" tractors.
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