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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.