I should point out that Magpul-style ladder rail covers used to be standard on all DD quad rails (and rifles/URGs with quad rails) up until early-2015 or so, when the "snakeskin" rail covers were first introduced.
Joe, correct me if I'm wrong, but didn't the ladders also come standard with all of the RIS II rails that DD sold to SOCOM? I always figured that's why they're so common in the pics I've seen of Block II rifles being used downrange.