This is both clever and super sweet of Downey, as that first arc reactor has come into play at very sentimental moments for Tony Stark.
Robert Downey Jr. goes on to caption the shot further with “proof that Tony Stark has a heart,” which is engraved on the prop he is holding, the very first arc reactor that saves Tony Stark’s life, and brings Iron Man into existence.
Downey is tying himself in with Stark in a tribute to the man who created his other half, while saying just how much Lee meant to him.