Miss Belvedere was a big prize
The car was advertised as an enormous gift. The game was to guess the population of the city of Tulsa in the year 2007. So the person who would be able to approximately guess the answer would get the car in that same year.
812 people answered the question, so 812 answers were put into microfilm and were kept in the glove compartment of the car. Then Miss Belvedere was put into the capsule with other 1957 memories of the state of Oklahoma. They made the capsule out of concrete to ensure that it would even resist a nuclear bomb attack.