Heli coil

That size reminds me of the british cycle thread. All diameters were 26 thread per inch. The Unified National Standard sizes are in between the UNC and UNF sizes and Harley Davidson is the only manufacturer that I know of that used them but they still would not have a 7/16 X 24 size.

There were different pitch sizes between 28/29 and 30/31 but they were all 5/16". What stanchion do you have there? A lot of these old cars have had the Rube Goldberg treatment at one time or another.
 
Unless it is a special bolt/head I would use the standard pitch heli coil and use a different bolt.
 
5/16 X 24 UNF helicoils are available from a lot of sources. The kit has all the stuff you would need unless you just need the coils. Coils are available separately too. Many chain auto parts stores carry them and there are several brands.
 
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