The knox form is the heavy part of the barrel where it screws into the action. Please see my post above where I posted two comparison photos of the MLE and SMLE knox forms.

I posted two illustrations to show the difference. The Magazine Lee Enfield (MLE, or "Long Lee") knox form (shown in the left photo) has a gentle "step down" contour. The Short Magazine Lee Enfield (SMLE) knox form (shown in the right photo) has a long, flat top surface.