A vent rib on a M21 seems right at home to me, but my friend ordered a CSM Fox with a M21 style vent rib and that seemed out of place to me.

I really don't care for the big ramp up in the back of the vent ribs I've seen on some Parkers. I don't know if they were factory ribs or done later. Looked bad to me.
http://www.bullseyesportingarms.com/photos.php?id=230&category=Shotguns