Quoted:
I must be dense, because I'm not getting something that is probably pretty simple.

The impact cover tail is supposed to be a go/no-go gauge to measure lip dimensions.  But when I try using it on brand new magazines, it slides right past what I think is supposed to be the "no-go" shoulder.  I'm sure I'm doing it wrong.

Can somebody explain how this works?  It looks like I need simple words and big pictures, because I'm not getting it.

You probably have an earlier cover that does not have the bumps either side to make it into a guage.