Sorry, Beardguy, I was interrupted mid-list, and edited my post to conclude it, after you posted.
You're right about Chemical King first appearing in-continuity in ADVENTURE 371, 8/1968 (by Shooter/Swan/Abel). That was the first appearance of Chemical King in continuity, introduced in the Legion's regular story-line.
The earlier ADVENTURE 354 appearance was in what was essentially an "imaginary" story, of the "adult Legion", that may or may not come to pass, and was just a commemorative statue of dead Legion members who died in action, in the distant future. Introducing a character that would be developed later.
But ADVENTURE 354 was still chronologically the first issue to have an appearance by Chemical King.
But continuity-wise, you're right, ADVENTURE 371 is where the character was fully introduced.
That was a tough one!
I just thought, that Shadow Lass appeared the same way. In ADVENTURE COMICS 354 (the imaginary "adult Legion" story) she had a cameo as "Shadow Woman" for a panel or two, as a commemorative statue in the future Legion "hall of dead legionnaires".
And wasn't mentioned again until she was introduced in the main storyline, in ADVENTURE 365.
The concepts for the characters were created, and then developed later, in both cases. But they were both dead before they were even introduced !
