Well - more or less, although they're really arguing that they're designed for both - 'dual' designed so to speak. That it's not really a modification, it's actually a design intended to be used for both in the same gun platform. While they use the word 'modified', what they seem to really be saying is that the new design was specifically 'redone' to deliberately suit either chambering and thus it was 'designed' for both.
If you actually modify a magazine to fit a different gun, then the 'original' design doesn't necessarily apply any more, but they're saying it's designed for both.
Now - i don't know if they're going to be able to sell that. It gets a little iffy. I think we would argue that ANY mag that works for 50 beowolf would have to work for 223 and it's not designed that way, it's just the way it is. And there is no way to make a 50 without it also being able to take 223 without a lot of special engineering. So - it IS a 'happy circumstance' that they work for both, not by 'design'. They argue it the other way around, that it's by design and not a 'happy circumstance'.
We'll see how it plays out. They are in a very grey area and i'm not sure how another authority will see it, although they tend to favour the police historically