Fantastic Four v5 #6

[Leoparis]

Per interviews with the artist the flashback is meant to take place during the Kirby era. I saw nothing that would contradict that placement. Actually I only saw a mention to #40 as the last time he was Ben Grimm, so this is before #78-79. They have their classic costumes. The only striking element is the absence of the Invisible Girl. So maybe this takes place after she left the team due to her pregnancy [#71], in particular when she relocated to a hotel [#73] and before Crystal joined [#81]. So the best place is right after #77. This puts the flashback very close to another story where Richards attempts to cure the Thing [#78-79], this time with a solution that deprives him of his power.