For your first few matches, while waiting for correct base plates to come in, we'd let you run CO...if you'd mentioned that they were over length by a hair. We do not verify competitors mag lengths unless it's grossly obvious. Myself and one other person in our club have the EGW gauge and mostly just make it available to members that are going to majors, so they may check their equipment and not get unpleasant surprise bumps to Open when on the road.
I, myself, have shot two level 2 matches where my mags weren't checked, and I'm thankful because they were just a hair over 141.25mm (that's why I got the gauge after someone else checked them for me at another local match). Surprisingly, the ones that are gross violators are the Sig P320s with their factory extended mags (lots of them in our club) and everyone has started shaving them down on the back edge to be legal.