(Believe it or not, there is a reason why such a post is necessary. Also, this is a followup post to these posts: one, two, three.)
This is not complete, but here is a rundown of some sources which are earlier than the 20th century.
From chapter 11 of De Humani Corporus Fabrica written in 1543, with a revised edition in 1555:
Here is a summary of ancient skulls, dug up, or found in mummies. "From Dental record: a monthly journal of dental science art and literature, Volume 4,"The number of teeth
Most often, the teeth [dentes permanentes] are thirty-two in number, sixteen in a row on either jaw, arranged like a perfectly aligned row of dancers. 11 Because the first four (nos. 1, 2, D) cut in opposition, they are called incisors, for they are broad and sharp so as to cut away readily by biting, like a knife, food put in their way, and break it up.
Note the two I underlined. Surely we should not expect a mummy to be Jewish! Nor is the skull of a Gurani, an Indian race in Paraguay, the skill of an Israelite. Yet, both have 32 teeth! Of course, someone dedicated to declaring that gentiles have 31 teeth or 32 while Jews have 32 will always have an answer. For example, that this evidence was planted by evil scientists in order to disprove the kabbalistic assertion. Or that when nishtaneh hateva hit, it also hit existing skeletons. Or that these must be members of the 10 lost tribes.
The following from the late 1700s, I think, from "The excruciating history of dentistry":
Note that he speaks of extracting 32 teeth, not 31 or 33.
The following summarizing a work by Pierre Fauchard, the father of modern dentistry, 1678-1761:
Thus, normally, there are thirty-two teeth, although there are deviations from the norm.
Here is another one, from "A history of dentistry from the most ancient times until the end of the Eighteenth Century". It discusses Galen's writings on teeth. Galen lived about the same time as Chazal, 129 -199 CE.
That is, Galen accurately described the number of teeth. He gives the correct number of incisors, canines, and molars. He lumps the premolars and the molars together. Usually, the molars of each jaw are five on each side. What does this mean? I believe that it means as follows: Teeth #4, #5, #6, #7, #8, on the upper teeth, and its reflection also on the upper teeth; and teeth #9, #10, #11, #12, and #13 on the lower teeth, and its reflection also on the lower teeth, for a total of ten teeth. Now, in some adults, the wisdom teeth, which are a type of molar, do not come in. This would then make only four on each side. Or, sometimes someone can have supernumary teeth. Modern dentists acknowledge this as well.
(The rest of the paragraph has to do with roots of various teeth and so is irrelevant to the present discussion.)
So this means that at least as far back as Chazal, the number and type of teeth was well known. And, it seems, the "standard" number of teeth was 32, just as it is today. And they were not talking about the dental configuration of only Jews. And we cannot say nishtaneh hateva, for this was known even back then. Similarly, it doesn't make sense to make use of "Rebbe-stories" (in the sense of stories about Rebbes) to prove the existence of anti-Semitic dentists who won't operate on patients with 32 teeth, which are anecdotal and may have changed or been reinterpreted slightly in the retelling. After all, this anecdote hits up against the established metzius, which may be confirmed by both dentists and historians. The same goes for repeated assertions of the difference in number of teeth between Jews and gentiles in many, many Jewish sources, be they Rishonim or Acharonim. This doesn't serve to prove the metzius! The metzius proves the metzius! The most it would do is undermine faith in all the Rishonim and Acharonim who asserted this, as well as make contemporary frumkeit a joke. (It shouldn't, but that it a tangential point.)