Steve, I just found this thread. Several interesting things. It's cool that you found card #1. I'm not really sure what to call them but I have heard to one being referred to as a broadcast card. I sometimes call it a body card.
Notice on the 2nd card that was found, it is NOT from your car! Really neat. It is from a SS coupe, whose body was built on the same day, June 9th. Now your case is a strange one. Usually when a mismatched card is found, it is from a car with the same interior. However, your car has black buckets and the other car had the rare Saddle bucket seat interior, which was not offered the whole model year. Typically, the cards were attached (with) the seats before the covers were put on. Once the cover was put on, the seats all looked the same so they would grab the seats to be for your car and lets say a bucket seat with a build card for the car built just before or after yours. It looked the same so they didn't know. Since the mismatched card was also from a June 9th car, the seat is obviously original to your car. Possible at the plant, the seat was to be covered in saddle vinyl but mistakenly was covered in black!
Who knows but it is fun to ponder! The 610 is the same on both cards and I think it has to due with the line/body jig the car was on when coming down the line.
Does your car have the original 396? Can you post the vin# and engine pad numbers? And do you have a better pic of the card from your car? I'd like to ad both cars to the database.