Losing the movie rights. Of course movie rights are not exactly the same as anti-fungal powder so we're a little confused by the publisher's response. We assume they meant that they lost some document since the rights are intangible. But what document could they have lost? The original publishing agreement may have spelled out who owns the movie rights. But if the book is out of print, chances are very good that the publisher no longer has any rights and that all rights reverted to the author. Publishing contracts form the 1970s often had "use it or lose it" provisions in which rights reverted if they were unexploited. Similarly, if the publisher assigned the movie rights to another entity -- usually as an option agreement -- there's a good chance that's expired and reverted as well.
Who's got the rights? Our guess -- and it's just a guess-- is that after forty years, all of the rights (including movie rights) reverted to the author. If the author is deceased, it went to the author's estate. You should contact relatives of the author to find out if they have a copy of the original publishing agreement. (That would spell out who initially controlled the movie rights.) If the author had an agent, perhaps that person can locate the contract. There are also pricey services that will provide a copyright search of titles. And of course, there are personal search services that can locate people. (The Dear Rich Staff used to use them to find absentee landlords whose tenants had frontal lobe problems.)