It’s hardly a proper Public Domain thread on a Star Wars forum without a Flash Gordon serial, not to mention the one with the most blatant tie to franchise.
Unfortunately I could’t find a better version of it on YouTube (the aspect ratio is somewhat cropped as well), however every episode is complete with intros and cliff-hangers, as opposed to the many movie cuts that exist of this particular serial. If you know of a better version online please send me the link in a PM.
Flash Gordon Conquers the Universe (1940 movie serial)
https://www.youtube.com/watch?v=pvNcU_eeXvg&t=2483s - from the Sci-Fi Hollywood Movies YouTube channel (3 hours 56 mins long - 1080p upscale)
Synopsis:
Flash Gordon, Dale Arden and Dr. Zarkov return to the planet Mongo for an antidote to the Purple Death, a virus that is wreaking havoc on Earth. However, Ming the Merciless has other plans for them.
Directed by Ford Beebe & Ray Taylor.
Cast: Buster Crabbe, Carol Hughes & Charles Middleton.
CHAPTER INDEX:
Chapter 1: The Purple Death
Chapter 2: Freezing Torture
Chapter 3: Walking Bombs
Chapter 4: The Destroying Ray
Chapter 5: The Palace of Terror
Chapter 6: Flaming Death
Chapter 7: The Land of the Dead
Chapter 8: The Fiery Abyss
Chapter 9: The Pool of Peril
Chapter 10: The Death Mist
Chapter 11: Stark Treachery
Chapter 12: Doom of the Dictator
Trivia:
It is this Flash Gordon serial that inspired the iconic Star Wars opening crawl. Every episode after the first begins with an opening crawl to catch up the audience on the story so far.