According to Wiki:-
"The head in the box ending was originally rejected by the studio but David Fincher was accidentally sent the original screenplay with that ending, convincing him to make the film. The producer still objected to it but Brad Pitt refused to act in the film unless the scene was kept."
No same "What's in the box?" ending = no Brad Pit
Apart from the box in SE7EN the most famous "What's in the box?" scene remains the one from KISS ME DEADLY (1955). Other "What's in the box?" moments are included in REPO MAN (1984), RAIDERS OF THE LOST ARK (1981), PULP FICTION (1994), CAPTAIN AMERICA (2011) and not forgetting BARTON FINK (1991):-
Brad Pitt, "With Seven, I said, ‘I will do it on one condition – the - - - - - - - stays in the box. Put in the contract that the - - - - - stays in the box.’ Actually, there was a second thing, too: ‘He’s got to - - - - - the killer in the end. He doesn’t do the ‘right’ thing, he does the thing of passion.’ Those two things are in the contract. Cut to: Seven has been put together, and they’ve tested it. They go, ‘You know, he would be much more heroic if he didn’t - - - - - John Doe – and it’s too unsettling with the - - - - - - - in the box. We think maybe if it was the dog's - - - - - - - in the box…"
David Fincher, "I just said, ‘Long after we’re all gone — 50, 60 years from now — a bunch of 20-somethings will be at a cocktail party talking about a movie that they saw on the late show the night before … the ‘- - - - - -in-the-box movie.’ I said, ‘This is the - - - - - - -in-the-box movie. Every person I’ve talked to … knows it as the - - -- - - -in-the-box movie. You can’t take the - - - - - - in the box out of the - - - - - -in-the-box movie.’ And he thought about it, and he kind of smiled and said, ‘OK. Do it."