From Doctor Fate: "The King of Crime" by Gardner Fox and Howard Sherman, More Fun Comics #76 (February 1942)
Half-mask Dr. Fate is the MAN! Well, obviously, I miss the total cray-cray of his magical adventures. It's pure Golden Age surrealism. But when I met Dr. Fate, it was in All-Star Squadron when he'd decided to forego Nabu's influence for a while. It made him more of a Superman-type, with enhanced physical abilities and not much magic, but I liked them exploring the dilemma.
And well, More Fun #76 was the last issue to feature him on the cover (damn you, Green Arrow!), so it's proper that I give him a little tribute here.
Comments
The half-helmet premieres in issue 72, as does his lighter, jokier personality (so those two things ARE connected). No explanation is given for the change.
So unless I find evidence to the contrary at some point, yes, we can assume this is a Roy Thomas retcon (from All-Star Squadron #23).