I'm pretty sure MPanova is correct, it comes with a short 10rnd mag to make the gun Mill Pro size, add the 15 rnd mag with it's spacer and you have a grip of a 24/7. Aslong as your not in Cal. anyway, my guess would be a compact 10 rnd mag and a full size 10 rnd mag for a Cal. gun.
If you live in California or a state that only allows 10-round capacity you'd be looking for trouble by doing a conversion.
You'd never be able to defend yourself legally with the mags in that altered condition. If you want it for range time you'd still have a problem if a LEO pulled you over and found it. (Felony)