First, do you have the PHS report? If not, then get one on your car. That will tell you everything you need to know to have one reproduced and I recommend AG Backeast to make the tag. There is rather limited info on those tags that year, even more so on a 68. In any event you can have a tag made to replace it but you first must spend the 50 bucks and get your PHS report for your car. Good luck.
1969 Firebird 350 Coupe, Rust free Texas body, formerly brown, now a TA clone with quality spoiler, latching hood and TA wheel; 1969 GTO Convertible Expresso Brown/Parchment(currently painted red). Both numbers matching. My first love was a Cameo White 67 326 Coupe, bought back in 1983, at the age of 17, for $1,200. Been hooked since on PONTIAC! Working on a 67 400 ragtop for customization-Gulf Turquoise/Parchment/400/4 spd! My winter project!