So what which car would have made it out the door first? Does your earlier trim tag indicate your car just left the fisher plant before his, but once it was at GM, his got done first? What is the build week of his car vs. yours?
The body number was assigned when the plant accepted the order for production, not when the body was built, so there could be a relatively short time span or a much longer span, depending on material availability, sales priorities, etc. Once the Fisher body was received at the Chevrolet Body Bank and the VIN was assigned, it would be unusual for the body release to the Chevrolet assembly system to be delayed more than a few hours.