ComicShelf.com has reported that the Internet Archive has made available early Phantom and Mandrake strips, as well as the un-aired Mandrake and Phantom TV-pilots from the 1960s.
The Mandrake content features 90 pages of strips as well as the stories The Cobra and The Weird Bank Robberies.
The Phantom content is much more extensive, including issues from Gold Key and Harvey Hits. You can see a full list of available content, as well as download it, from The Internet Archive.
Thanks to Ben DeHaan for this info.
