This is the smallest one I have. This is a 0000-160 tap. That is a nickel it is sitting on. It makes a threaded hole .020" in diameter and 160 threads per inch.
Some of the metric sizes are so close to the TPI threads that it is pretty tough to tell with a thread gage alone. I have checked thread gages on an optical comparator and most are pretty close but the cheaper ones tend to be just sort of "in the ballpark" dimension wise. Because of this, the only method I really trust to determine the pitch of a mystery thread is to use an optical comparator to count at least 10 threads and then calculate the pitch. (The more threads you use in your calculations, the less error per thread.)
I have been out done! My set is for watchmakers. It is quite amazing to me that in order to make small machines, like watches, someone has to be able to make tools. In the 19th century, the watchmaking companies had belt driven equipment to make tiny screws by the spoonful. They were remarkably uniform and often had polished heads, many were also blued.
The US Navy used to have a rating called Opticalman (OM) which came into existance around 1948 and was abolished several years ago. OM's were the navy's repairmen for binolulars, telescopes, gunsights, periscopes, compasses, sextants and various other navigational instruments as well as any other thing that there was not a particular rating to do the repair of. During the initial training school for OM's was a machine shop course whereby the trainee was required to perform numerous machine shop operations from simple finishing rough metal with a series of hand files up to lathe, mill, and shaper operations. They were required to demonstrate their prowess by making special tools which would be required in the future performance of their duties as OM's. These tools consisted of, from a simple screwdrive,r to a "pin wrench" used to remove threaded retainer rings with either slots or holes in their faces. Amongst these tool was a "Screw Gage Plate" that had threaded holes from 0-80 thru 1/4-28 in both UNC and UNF and unthreaded holes of tap drill and body drill sizes that would correctly identify all the threads in this range. I guess the intent of this post is to say that there are many, many of these tools out there in the hands of ex or retired OM's that might be obtained by contacting one of these people. There is a Facebook group entitled "U .S. Navy OM/IM" that can be accessed to connect with people who may or may not wish to turn loose of their tools.
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Don't forget that along with UNF and UNC there are a series of threads labled "National Special" as well as "National Gun". And I am sure there are at least a few others. Machinery Handbook is a good source for information that most of us consider obscure but none-the-less informative.
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