My read was that they felt that it was originally designed for 223,
and that it was subsequently modified/adapted using one of 3 techniques ("widening the space between the magazine lips, changing the angle of the magazine lips, or changing the feed angle of the magazine follower") to feed 50 Beowulf without deleting the ability to feed 223. This makes them prohibited.
That how they are currently is that they are now designed as dual caliber, regardless of what's printed on them, which also makes them prohibited.