In Avengers: Endgame, after
Loki makes off with the Tesseract in 2014,
Tony Stark says to Steve Rogers that he knows
when they were both there, and you know why I know
Referring to Hank Pym and Howard Stark.
Then the two of them
travel back to 1970 to obtain the Tesseract and more Pym particles from Howard Stark and Hank Pym, respectively.
What was Tony referring to when he said this to Steve?