59 are screwed on. What was wrong? Metric screws? If it was pop riveted on I'd say bad restoration.
Not sure but it was the law that saved the car from the crusher. Searching 59 Vettes that have sold at auction I found the "common" screw used but also found a car with rivets, another with flange head screws so I'd say they cops had their mounting fastener books available when going over that car.