What makes this model unique is the handle which is different from the second and third model of D20 series.
The distinguishing characteristic is the notch which is pointed out at right. Saws with this feature are very rare, and it is my estimation that because of this fact they must have been made only for a year or two around the start of production in 1911.
The other thing which is unique from the second model is the handle cutout is asymmetrical. As can be seen in the photo immediately below, the cutout on the second model is more elliptical in shape.
These differences are subtle, but a careful inspection can tell you if you have a first or second model D20 series handsaw.