Yep! Namor the Sub-Mariner first appeared in the venerable golden-age Timely Comics title Marvel Comics #1 way back in November 1939. And then, after the old company eventually took the operating name of Marvel Comics, the Atlantean Prince Namor famously reappeared in Fantastic Four #4 in May 1962.
So, why is Sub-Mariner the subject of a 70's Flashback?
It's that spiffy 2nd costume of his that's why; which was introduced in The Savage Sub-Mariner #67 (November 1973). 'Twas quite a change from the decades old swimming trunks that Subbie had worn for the previous 34 years, to suddenly have him wearing an almost full set of clothes. He was a seafaring hero after all.
But therein lies the rub, you see although certain elements of Namor's persona [which have a direct connection to this costume] were not completely explained until John Byrne's early 1990's series Namor the Sub-Mariner, Byrne nicely tied into the original reason offered for the costume change - and here my memory is a bit fuzzy - suffice it to say that Byrne posited that as a hybrid Atlantean/human Namor's biochemistry causes him to "freak out" after being away from the ocean for too long (and vice versa the normal surface world, when he has been ocean-bound for too long). I think that the original explanation was that the 2nd suit allowed Namor to maintain a close contact water source (within the costume) to minimize the loss of his enormous strength - - - which typically faded the longer he was away from contact with the sea.
Either way, I always thought the costume was cool and it still pops up on his mutant frame from time-to-time.