The regulations are here:
FCC communications Regulations
And they are byzantine.
However, I find that several years ago, under "de-regulation" they dropped the requirement to supply the broadcast logs to the FCC. EXCEPT, they must log political ads and proofs:
"Occasionally, I still get questions about program logs. However, there is no longer any requirement to keep program logs or records of material that is broadcast, whether it is commercial matter or programming. The exceptions to this, of course, are the public file requirements for the quarterly report of issues and programs and the political file required in each station’s public file. However, the public file is separate from either the station log or a program log, neither of which the public has a right to see."
However, many stations are maintaining such logs and retaining them because they become important ammunition when re-licensing comes around. They have been instrumental in many stations ability to retain their licenses in the face of community group challenges claiming non-representation or claims of discriminatory advertising.
Right, so there’s no requirement to tell the FCC who your advertisers are, except for political adverts.