Roughly around the same time, Filmation tried an adaptation of the legend of King Arthur, which would've been the first series to adapt the Knights of the Round Table in an adventure series. Ken Sobol, principal writer for Journey to the Center of the Earth, which premiered in 1967, and Fantastic Voyage (1968), wrote this pilot, in which Arthur (Marvin Miller, the voice of Aquaman) must rescue Princess Guenivere (Jane Webb) from the Black Knight. Webb would also voice Morgaine Le Fey, and subsequently recycle the Le Fey voice for Catwoman (The Batman-Superman Hour).
Most scholars claim this was made in 1967. However, there are a few clues that suggest that this was later, even though the copyright date looks like it was made around 1964 or '65. I'm guessing that the cartoon was actually made well before the studio signed the contract to adapt the DC heroes (Superman, Batman, etc.), but it remained in the vaults until Filmation established itself on the air. Additionally, this would be the first instance where director Hal Sutherland's signature appeared on screen, as well as the circular credit of producers Norm Prescott & Lou Scheimer. These particular items didn't begin to appear on Filmation programs until 1969 (i.e. Hardy Boys). Finally, Webb didn't make her official debut with Filmation until 1967 (Journey to the Center of the Earth), and remained with the studio until 1977.
Edit, 12/1/15: I've since been apprised that the copyright date is 1974, which meant this project took 7 years to complete. Many thanks to Doz Hewson for providing some assistance.
Now, let's scope out King Arthur:
While the artists weren't credited, the linework seems to be an attempt to emulate the work of Hal Foster of Prince Valiant fame. One wonders if Foster wasn't somehow involved in the production of this piece.