Re: EREK or US #3 M40
The EREK (stands for Erector REpositioning Knob) is a work of art. As the name implies, it allows you to reposition the erector, actually moving the reticle in the scope. This allows you to zero the reticle to the bottomed out knob, preserving full elevation travel. Some argue it's a love or hate look. I love the low-profile look and it's an ergonomic joy.
The M40 knob has no zero stop.