When the mag release on Gen 4 models is reversed for left handed operation, You must use the Gen 4 mags for them to lock in. These new mags have a additional notch on the other side of the mag for the relocated release. Older mags will not lock in. Store inventories will probably be without these "New" mags for awhile and everyone will want these updated ones, 20 and 30 rounders even longer.