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!
