To add to
@four.cycle 's hardware store ad citing "1932 Chevrolet Special" for this axle size and thread, here are some excerpts from a contemporary K-D catalog...
...identifying it as the "BA" (or "Confederate"* model) series of cars.
Not sure why you didn't look anywhere else before arriving at the prototype/race car theory or letters to museums, but note that you can also readily find 13/16"-16 knockouts in late 1930's and early 1940's catalogs from New Britain, Herbrand, and other mfgr's.
If you look back in the 1928 Bog catalog, you'll see that they were only offering four (4) sizes of the "Drive-Out" or "Knock-Out" type pullers at that time. By 1932, which is the catalog your page is from, they had added four (4) additional sizes for both type pullers, but it was almost certainly published before the '32 Confederates* were introduced. Unfortunately, the 1937 Bog catalog does not include either of these type pullers at all, but
@four.cycle 's ad shows that they had the 13/16"-16 covered in their lineup of "Knock-Out" type pullers by 1935, and your example demonstrates that they were offering it in the "Drive-Out" type as well.
[Note that this post was made without posting any wonderful but completely irrelevant examples from my collection of vintage knockouts from other mfgrs!] (...inside joke...)
* I would love to see Detroit (or Seoul or Aichi etc) just try to name a car a Confederate today!