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If you don't get a solid answer, you could take the nut off and take it to the nut/bolt section of home depot/lowes and use the thread and size finders they should have on the walls in the area. Should have a section with labeled bolts poking out.
If you are planning on using an eye nut for a racing harness attachment you should make sure it can handle the shock load. Not sure what the Metric number is for a Grade 8 equivalent but it might be hard to find one like that.
Designer Imagines A Corvette That Looks More Like a Corvette Than the Corvette
Slideshow: A Jaguar designer's personal project imagines what a modern front-engined Corvette might look like if Chevrolet revisited the golden age of the Stingray.