Cropped screenshot of Katharine Hepburn in the trailer for the 1975 film Rooster Cogburn

Public domain explanation

• Trailers created prior to 1976 were copyrighted under the Copyright Act of 1909, which determined that in the 28th year, the owner of the copyright had to renew the copyright. If he did not, his work went into the public domain (see here).
• The copyright for this trailer would therefore have to be renewed in 2003. Searching for "Rooster Cogburn" with the United States Copyright Office reveals that the movie has its copyright renewed, but there is no evidence that this was secured for the trailer.
• It is possible that copyright was never properly secured for the trailer in the first place. Under the 1909 Copyright Act, which was law until 1978, copyright required 1) publishing the item with a copyright notice; 2) depositing two copies of the item with the Unites States Copyright Office (see here). Creative Clearance notes that most trailers were not registered with the Copyright Office in this way. The lack of a copyright renewal for this trailer, despite the fact that the movie secured it, suggests this may have been the case.