I read "The duration of the U.S. protection for all other works… was for 70 years from the artist’s date of death" and thought wow, did Mondrian really live into the 1960s or so? Next paragraph: "Mondrian died in 1944. Any of his works subject to a life-plus-70 regime would have entered the public domain" 10 years ago. Who even thought of including that in a legal argument??
There's all sorts of quirks for anything published before that rule got standardised more-or-less worldwide, but in general 1930-1945 is still like a legal grey area that can be challenged in court and you should be good to go for anything published before that. And don't get me even started on posthumous publications, that's a whole different can of worms where a family member might claim some contribution (like for example Anne Frank's father), pushing the copyright further to the life of the author + life of that family member + 70.