In the "Standard Tread" configuration, the tractor has a wide front end which is NOT adjustable for width. When the model is ONLY available as a standard, generally the rear wheels are not adjustable either. The "Styled D", pictured above, is a good example.
The tractors that were available only as "Standard Tread" were the D, BR, AR, R, 80, 820, 830, 320, 330, L and LA.
In addition to having the usual tricycle front end configuration with TWO wheels close together, Some of the row-crop tractors were available as "Standard Tread" tractors. Notice that this Model 60 also has fixed-width rear wheels.
Notice that this 720 "Standard" is actually a rowcrop tractor with adjustable rear wheels. Notice, though, that the front end is "Standard" and is different from the adjustable wide front Model B that is shown below.
Just as a reminder, this is a late styled Model BW. It is a "row crop" tractor. The width of BOTH the front tread and the rear tread are adjustable. The Model AR in the background is a "Standard" tractor.
The Model H, and the successors to the B - the Models 50, 520 and 530 were NEVER available in "Standard Tread".
When the Model B appears with a "Standard" wide front end, it is known as a Model BR. Back then it was known as a "Regular" tractor. In the case of the Model BR and the Model AR, it really IS a different model than the tricycle. In some of the other later models the distinction is a little more hazy as you saw up above with the Model 720.