I live in the world of old movies, all the way back to 1929 or so. But you only have to go back to the 60's before you start seeing a large number of "unrated" movies in TMDB because there is no support for the assigned ratings. I have seen some "Approved" movies being given a "G" rating in TMDB, but they are not the same.
A pretty good summary of the ratings used in the US over time can be found at: https://en.wikipedia.org/wiki/Motion_Picture_Association_of_America_film_rating_system. It does not address the earlier rating of "Approved" (or "Passed"), used to indicate that a film was compliant with the "Hays"(pre-1968 code).
I suggest adding the tags: "Approved" (Hays) (I have also seen "Passed" used) "M" (mature) "SMA" (Suggested for Mature Audiences, but rarely used) "GP" (more or less replaced "M") "PG-13" (more or less replaced "M") "X" (more or less replaced by "NC-17")
Hopefully I haven't missed anything.