I'm pretty sure only gen4 mag releases are are ambi. And you can still use older mags in a gen4 long as mag release button hasn't been reversed to the right side.
The knotch is in the front of the magazine, not the other side ridge. Easy modification. The knotch will not interfere with the operation of the mags in your std G21. Don't overthink this... They're just plastic and metal, hell an exacto knife could cut the "window" needed.




























