Any works that did not carry a copyright notice in the credits prior to 1974 were considered to be public domain. As far as I understand, some TV movies are available on the Internet Archive under that premise.
After 1974, creation of the work was enough to grant automatic copyright, no notice required.
As a preliminary matter, I have correct some of the facts you mentioned.
Copyright has always automatically existed in original creative works from the moment they are fixed in a tangible medium of expression (ink on paper, magnetism on tapes, grooves on CDs or records, etc.). Before 1978, protection was under state law. The 1976 Act, which went into effect on January 1, 1978, abolished state-law copyright (also called common-law copyright) for all works except for sound recordings fixed prior to February 15, 1972 (when federal law began protecting sound recordings).
Before March 1, 1989, there was a notice requirement to maintain copyright. Before 1978, published works were required to have a valid copyright notice on each copy. However, the definition of published under case law excludes at least network television broadcasts, as the tapes are not generally distributed to the public (The leading case involves the first season of
Star Trek, which has no copyright notice). After 1978, the definition of publication changed in a way that could include distributing a copyrighted work to more than one station for broadcast. However, the notice requirement also changed. 17 USC 401 requires a copyright notice on each copy of a copyrighted work distributed to the public. Broadcasts are not copies, since they are not fixed, and the tapes used for broadcast are not distributed to the public. Hence, after 1978, the notice requirement for television shows only triggers upon distribution to the public of actual copies of the works (such as on home video).
Regardless, unpublished works never had to have notice under copyright law.
In light of this, I stand behind my opinion that the only game shows that are in the public domain are the ones that registed for federal copyright prior to 1964 and then did not timely renew the registration in the 28th year after the original registration.
