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.

I learnt that the Se7en OC box actually opens like a cardboard box

Se7en / Seven Ending Explained (A Detailed Plot Analysis)

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

Seven - What's in the box ? ending - Full Scene - Se7en - Vidéo

SE7EN Scene - The Box

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

In this gut-wrenching scene from Seven, the director wanted to

Se7en Ending Explained (In Detail)

Seven (Se7en): What's In The Box Special Edition