Is "The Naked Kiss" Missing Copyright Notice?

Criterion's pristine version
of "The Naked Kiss"

Dear Rich: I'm having a debate over the legitimacy of a chain of title of the motion picture, "The Naked Kiss" (1964). There are very clear records in the post-1978 Copyright Office database indicating copyright ownership but this person is insisting it is an invalid copyright because there is no “©” insignia anywhere on head or tail. I’ve found a lot of confusion about this, but my understanding is that even without an insignia, its creator could have copyrighted it. Does the absence of the insignia mean that the film is public domain?
Nowadays, the publication of a film without copyright notice won't effect ownership, but in 1964, the penalty was draconian. If the copyright owner failed to include a copyright notice on "The Naked Kiss," then it would likely be in the public domain. This was the rule until March 1, 1989, when the notice requirement was terminated.
Is the copyright notice missing? Distributors of public domain films have considered "The Naked Kiss" as PD for at least twenty years. Ditto for online sources such as the Internet Archives. The IMDB entry states, "No copyright year is shown at the beginning/end credits." [Link may require a subscription.] A proper copyright notice includes (1) the copyright symbol © or the word, "copyright", (2) the year of first publication, and (3) the copyright owner's name. Under the copyright law effective when "The Naked Kiss" debuted, a notice was defective if the date was missing.
What about the copyright records? Documents filed at the Copyright Office as recently as 2011 indicate that various parties controlled copyright to "The Naked Kiss". Yet, if they control copyright why do they permit so many vendors to distribute the film freely? It may be because a lawsuit over the film (we couldn't find any) would officially designate it as PD thereby denying future licensing revenue.