I keep finding these fancy grade 9 bolts so I decided to see what they were used on back in the early 1960s. Turns out they were used on race cars not military aircraft which was one of my earlier guesses. The 2 big ones (7/16-20 UNF) are rear axle adjusters. The 3 1/4-28 UNF ones are for the right front motor mount and the carb to elbow location.
Yeah, I am a bit of a hardware freak.