I'll bet there's some secret organization out there, something like, World Council of Progress, whose secret Museum of Progress houses all these revered missing source materials. Such documents and media are priceless as they chronicle America's Long Descent into socialism. Especially to the elite and benevolent first among equals who grace the halls of such a museum.
I'll further bet the real reason centralized (and thus monopoly) government marks certain documents for release only after 50 years is so they can first reside on loan in secret Museum of Progress.