This is an odd thread - finer than fine. This would probably be used in an application that is subject to a lot of vibration.
Here are the 7/16 bolts from the 1959 K/XL Spare Parts Catalog. Note that in this, and most Harley catalogs, the tech writer has capriciously misapplied the N.C. and N.F. labels. 7-16"-14 is National Coarse, and 7-16"-20 is National Fine. Ignore the abbreviations, and use the Threads Per Inch.
- Note capricious misuse of N.C. and N.F.
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National Coarse was formerly known as U.S.S. and National Fine as S.A.E.
Harley also uses the N.S. abbreviation for some obsolete threads ("National Special"), of which they use very few. One is 1/4"-24. (1/4"-20 is N.C. 1/4"-28 is N.F.). Bolt 2768 and nut 7691 were used to attach the 1952-1955 taillight to the fender. There are a couple of other instances of this thread. These were standard many years ago, but are long obsolete. But Old Dude still sells them.
Most other weirdo bolts and nuts get a Harley number, and they don't list the specifications.
What do your bolts fit? What's the last piece of antique farm machinery, airplane, industrial pump that you took apart?