Inside the case's cover, alongside the keyhole, look for tiny bumps, called wards. (See Step 7 photo.) Pass the bit through the keyhole and mark where it contacts the wards. Then insert the key's tip from the opposite direction and mark where the wards hit the bit's other edge. Clamp the key in a vise and file two notches in the bit, as shown.
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Bench grinder (optional)
1. Penetrating lubricant, such as WD-40