Affidavit of copyright claimant

What does Affidavit of copyright claimant mean in American Law?

The definition of Affidavit of copyright claimant in the law of the United States, as defined by the lexicographer Arthur Leff in his legal dictionary is:

Under the “manufacturing clause” of the Copyright Act prior to its 1978 amendment, severe disabilities attached to copyrightable works manufactured (e.g., printed) abroad. Hence as part of the copyrighting process, the applicant had to submit an affidavit swearing to U.S. manufacture. The new Act contains a far less stringent manufacturing. clause which, moreover, is to expire on July 1, 1982.