What was actually in the box in 'Seven'?

In the terrifying final moments of 'Seven', David Fincher intentionally places us in Brad Pitt's character's position, refusing to let us see what's in the box.

In the movie Se7en, what was in the box? - Quora

Preorder the Se7en 4K UHD and Blu-ray Collector's Edition - IGN

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WHAT'S IN THE BOX?! - SE7EN / Seven Ending - Full Scene

What's In The Box? The Shocking Se7en Finale Which Was An Accident

In the movie Se7en, what was in the box? - Quora

Mystery of eerie 'what's in the box' Se7en ending revealed - and

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Spoiler] Se7en (1995) The box sticker Please handle with care

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Se7en Film Locations

Se7en™ - What's in the Box? Poster