I imagine that the person who wrote "that most publicity photos prior to 1964 would be in the public domain" was talking about the situation in the USA where at that time a work needed to be registered and carry a copyright notice (something like "Â© John Doe 1964
") in order to be entitled to copyright there.
But it is a big assumption to say that 'most' publicity pictures would fall into the category where these things weren't done. As 'most' such pictures would be produced by professional photographers, I think the opposite is true, because they would value their work and want to protect it. If an image wasn't registered and didn't carry the notice, it could be copied straightaway, so obviously a photographer who didn't take advantage of the copyright system stood to lose licence fees from these copies.
Initial registration lasted for 28 years and so images from that time which were correctly registered and had notices would still have been in copyright at the time the 1976 Copyright Act came into force in the USA, changing the term of copyright to the lifetime of the author plus fifty years (later raised to 70 years).
As the situation regarding pre 1976 works produced in the USA is immensely complicated, depending on whether something was registered and then whether in some cases, the registration was renewed, I would advise you to use a site like this one at Cornell University
to help work things out. If you are patient and diligent you can use* the online US Copyright Office registers
to track down whether something was registered, although since the registers don't show the actual image that wouldn't be very useful in this particular case as there will probably be dozens of pictures of many of the stars of the day registered, and even if you know the name of the photographer, that won't necessarily narrow it down sufficiently.
* I suggest that if you do decide to try this, you should read this helpful guide