In the old days, before the sunset of the 1994 ban, we used 10-round .40 cal mags in lots of 9mm guns. I had .40 mags for HiPower and Beretta with slightly modified feed lips that held 13 and 15 rounds of 9mm, respectively. They worked perfectly.
This is similar in concept, but the weird part for me is that they seem to be structurally identical to the 3914 magazine. I dont have any .40 ammo to check, but it appears improbable that it would fit in these mags. Of course thats nonsense, but 9mm fits perfectly.
I swapped for 3913 followers in all of them as an afterthought, but they may have been totally unnecessary. They worked just fine with the .40 followers.