IRON MAN 55 and 56, Feb and March 1973, what was just another fill-in assignment evolved into Starlin's big break. And remarkably early into his time at Marvel, as Starlin's first published work for Marvel is cover-dated Oct 1972.
First IRON MAN 55 and 56, then CAPTAIN MARVEL 25-34. And at that point, Starlin had already achieved godhood among comics fans.

And a few issues into his CAPTAIN MARVEL run, two issues of MARVEL FEATURE 11 and 12 (two Thing team-up stories), sometimes left out of reprint collections.