Sorting hardware the other day I found this odd nut, shown here between a couple of normal ones. I don't recall ever seeing one like this.
Looks like a Mexican knockoff...
Fine thread vs course thread? Paul
My wife says I'm a rare nut.
Ahh It just needs to be cooked until it is medium well
Off a Stovebolt.
Depending on the bolt diameter, the 1928 to 1938
Ford parts book lists 16, 18, 20, 24 and 28 TPI
for that style square nuts.
I wonder if it's been re-tapped?
Also, prior to about 1900, there was no thread size or count standard for nuts and bolts. Many were still hand made by blacksmiths. Most were manufactured by companies that made them as matched sets to their own standards. There were several common near standards that different companies or individuals would work close to. Most worked close to even common fractions of an inch. It was very common for nuts and bolts to be close, but not quite fit together.
I have a bunch of old hardware, some of it belonged to my great grandfather. Fortunately, I also have some of my great grandfathers blacksmith tools and taps and dies. I sometimes have to use them to refit really old nuts and bolts.
I would not say that what you have is or is not really early or later. Just something to consider.
Drive carefully, and enjoy, W2
In modern parlance, that's called a Flint Nut, as in Flint Michigan. In later years, they were spun welded onto fenders for machine tool assembly. Wrong washered bolt and theyd break loose and spin being heck to remove inside of a fender.