Lots of stamping, mostly at odds with itself.
1. Never known a racer in my 50+ years around or in the Sport that would stamp something like that whole term; much more likely to use "GN". Sanctioning bodies use the same shorthand.
2. Grand National term itself changed after the '71 season, when it became Winston Cup, with the advent of series sponsorship.
3. The date stamp seems like an engine rebuild date, which I guess someone might do.