I saw a comment from magnetic_regina asking whether the Debut of Doom had been posted, and amazingly, it hasn't. Rectified!
"Prisoners of Doctor Doom!" was published July, 1962. After much deliberation, I narrowed down the preview image to just one of the many crackalicious panels:
'And I'll call myself... Mister Fantastic!!'
Apropos of nothing in particular, a preview image: