The BBC has just released a documentary on “The Day The Clown Cried”. Make sure you see at 9:01, where never before seen photos from the set are shown.
Donations to museums and archives often come with those kinds of stipulations. Waiting X number of years before making the donated material available is actually pretty darn common. In my experience it really doesn’t have anything to do with who has rights to the film. It’s simply an agreement made by the institution as part of accepting the donation.