What additional laws or rules apply after ipo to stock that was
aquired before a company is public? Please cite specific sections of
the law. (I'm aware that underwriters can impose additional rules, but
I'm specifically interested in SEC rules)

Are owners of pre-ipo shares (including non-insiders) required to
disclose total number of shares owned, or other non-standard
information, when selling those shares in the public market? If so,
please locate this information for a recently IPOed company.