Mr. Fantastic has two flashbacks from FF 555 in his chronology. One in his early days and another right before the main FF 555 story.
Alyssa Moy appears in the first flashback, and Thing appears in the second. However, Thing is listed as appearing in the main story; he doesn't, he's only in the flashback.
MR. FANTASTIC/REED RICHARDS
…
BFF:RR 3-FB
*FF 555 (1-2)-FB
FF 513/2
…
FF 554
*FF 555 (12-14)-FB
FF 555
…
THING II/BENJAMIN J. GRIMM
…
FF 554
*FF 555-FB
FF 556
…