As to the feed - that is why I believe the guide-lip is different, since the cartridges are coming in a precise path, as opposed to being tossed at random, the lip can be higher and more beveled to translate the downward push to a sideways slip.

As to the rules, I do not know - I'm fairly sure they'd have to accept a real Parkhurst, but would not be too surprised if they balked at a "Steinmayer Device".  :-)