Except you can buy these legally, unless you have a PMAG that is between 10 and 15 rounds stock (I'm not aware of one).
It is illegal to buy a 16+ round magazine now.
It is illegal to convert a 15 or smaller round magazine to over 15 rounds.
These could be legally used to make a 10 round pmag hold 15.
There is nothing in the law about making a grandfathered 16+ round mag hold even more.
So if you have a "high cap mag" it seems perfectly legal to make it even higher capacity, so long as it was already over 15 rounds and you owned it before June 01 2013. So you could have a 30 round grandfathered PMAG and legally make it hold 35 with this. Or a 20 -> 25 or 40 -> 45 also.