Bullet Button for AR-15
This is the thing that makes a standard AR-15 legal in California. It replaces the standard mag release to force you to use something small (such as the tip of a bullet, hence "bullet button") to release the magazine. Just another way California tries to reduce crime by unnecessarily punishing law-abiding gun owners.
EDIT: I asked a friend in the DOJ if putting an "adapter" that fits in the hole to basically bypass the Bullet Button would be considered illegal, and she told me it WOULD BE ILLEGAL to do so, because if it bypasses the restriction imposed, most LEO's and the DOJ would consider it a felonious alteration, incurring serious assault weapon charges.