I don't actually know, but I do recall hearing that old SAE fasteners had ridiculous converted metric specifications in some systems. So instead of using a 3/8-16, it was a 9.52 x 1.587mm or somesuch.
I'm not sure if american aviation uses ISO, ANSI, SAE, or some other standards body to define specs for fasteners. (AIA?) It's also possible that there's multiple specs in play.