Hi, I couldn't find much information on your question. But I did find this post from two years ago made in this subreddit.

The top comment contains a broken link, but I found a press release and a news story about it.

Hope this is what you were looking for!

The US classification system as you think of it is not that old, relatively speaking. The ability to declare that a piece of paper is "secret," meaning that it can only be accessed to people with certain official authorizations, and must be kept in safes or guarded by people with guns, and with penalties for anyone who violates these rules... this only dates to around World War I. Prior to World War I, there were some vague regulations in a military context about giving away information about fortifications, but the modern US secrecy system starts up in 1917 with the passage of the Espionage Act, which is the first law that gives teeth to the idea that you can prohibit the transmission of information.

Even then, several more components of the "modern" system did not fall into place until World War II and the immediate postwar ("declassification" procedures did not get created until late 1945).

So you aren't likely to find anything legally secret prior to 1917. This does not mean that they did not keep things informally secret prior to 1917 — there has always been informal or bureaucratic secrecy. But this is not the same thing as something being legally "classified." (In my own work, I identify four essential components of any true "secrecy regime": demarcation of information, demarcation of people, handling guidelines, and legal consequences.)

Separately, it was not until around World War I that the classification of technical information really began in the US. Technical secrets are the only secrets that can potentially have no expiration date (unlike military or diplomatic secrets, which become increasingly stale over time). If one were looking for "likely candidates," things related to cryptography, steganography, and chemical warfare are what one would suspect most of all. The only other category of data I would expect to be classified in some way for ages and ages are the names of confidential informants (for the FBI), and, more and more today, anything that might fall under the heading of "privacy violations" (which has become a convenient way for government redactors to avoid telling you who was doing anything in the past, as an aside).

A very nice legal overview of the US classification system is Arvin Quist, "Security Classification of Information — Volume 1. Introduction, History, and Adverse Impacts".