Just to clear things up. It was second class that was newspapers and periodicals. You had to apply for a second class mailing permit you couldn’t just start using it, though.
Third class was “unsealed” but actually it could be selead, though the post office could break the seal and inspect it if it wished. (I don’t believe it needed even probable cause, but I’m not positive.) Postcards, of course, were unsealed, but otherwise handeld like first class.
Fourth class or parcel posts was packages over one pound in weight. Under that they were also third class.