My 62 that I had had one key that fit everything.My '65,66,and all my Chevelles,Monte Carlos and El Camino's had a separate key for the ignition lock and the doors. The trunk and the glove box had a separate dedicated key different from the ignition switch lock. We had a customer come in one afternoon with a fairly nice 70 Chenille wagon that had the door lock cylinder missing. He came out one time and he thought that it had just fallen out. We found his key code in the vehicle's driver's instruction manual in the glove box, and we made him a new lock cylinder with the little pawl and clip that hooked it up to the for latch assembly inside the door. It occurred to me after he left, it's possible someone used a slide hammer to pull the door lock, and have a key made at a hardware store or another dealer's parts department so he could go back and steal the car with his new key. I found the flat steel horseshoe clip the held the lock cylinder into the door inside the bottom of the door. I don't remember them having much tendency to fall off,either. Buying a replacement ignition lock and tumbler assembly and putting it in would have been a good way to thwart that, since the replacement tumbler would have had a different key than the door locks. Another thing with pre '65 GM cars was the lock that had a Lock Off On Start position on the ignition switch. If somebody didn't lock it, anybody could get in, start the car and drive off with it without keys. Not good.