Hi,
One way to know if the coin is indeed one struck on an incomplete planchet is to look for what is known as the "Blakesley" effect. Go To:
http://www.triton.vg/clip.html for the general idea. The explanation is not too bad on this page. Your coin shows this effect. Also, the lettering on the area near the "clip" will look, as it does on your coin, as though the lettering flows off the coin. That is different than a forgery of this error type where it is common for the lettering to look as though it just suddenly stops at the point that is "clipped".
Have Fun,
Bill