One doesn't normally think of corporations as being agencies of the United States government. However, the California Corporations Code conclusively presumes that some corporations are agencies and instrumentalities of the United States. To qualify for this presumption, the entity must:
- Be a corporation organized under the laws of the State of California, any other U.S. state, the District of Columbia, or an act of the U.S. Congress; and
- All of its shares must be beneficially owned by the U.S., an agency or instrumentality of the U.S. or any corporation the whole of the capital stock of which is owned by the U.S. or by an agency or instrumentality of the U.S.
Cal. Corp. Code § 103. A corporation meeting this condition is entitled to all privileges and immunities to which the holders of all of its stock are entitled as agencies of the U.S.
I don't know why the California legislature thought it was necessary to address this question. I do know that Section 103 was carried over from a provision of the prior California corporations law that had been on the books since 1947.
The Public Company Accounting Oversight Board would appear to be an agency or instrumentality of the U.S. because it was organized by an act of Congress (Section 101 of the Sarbanes-Oxley Act of 2002). However, the PCAOB is a nonprofit corporation without shareholders. More importantly, the statute creating the PCAOB states "The Board shall not be an agency or establishment of the United States Government".