Justice league 20 - What was in the box?

What do you think was in the box that Batman gave to Superman? The one with the bat symbol on top to signify a way to stop Batman.
I first thought it was symbolic, 'cause superman doesn't really need help to defeat him in H2H, but he looked shocked when he opened it so I'm not sure, what do you think?

Here's a scan on the scenario:

It is probably a self destruct button similar to the destruct button amanda waller uses on the suicide squad members.

Its his moms pearls

Batman is just a human and should be easy to kill by the police or the military if Batman ever becomes evil.

A picture of his parents.

proof of his secret identity? self destruct button for his suit and the batcave?

Anti-Batman Spray?

Bomb filled with kryptonite? That way if Batman goes bad, Superman will open the box and trigger the trap, killing Superman instantly. It's so evil, it must be true.

It could also be Bruce Wayne's secret allergy like maybe wheat gluten.

His circumcised foreskin

Maybe it's:

• A gun (HIGHLY unlikely.Because bullets shouldn't be that much of a problem for him).
• Self Destruct button (nanites like with Amanda Waller....however if that were the case that it just were a switch/trigger,I doubt that Clark would have that reaction or even know what it was for).And perhaps if he were evil he would be able to destroy them
• Some relic from his parents murder,in which it would be a psychological weapon instead of a physical one:

1. Thomas Wayne's letter
2. His Mom's pearls
3. Joe Chill's gun
4. OR maybe it could have something to do with DAMIAN WAYNE.Maybe his mask,or hood/cape.
5. A photo...but It's the least likely one...even under Joe Chill's gun

Based on the shocked face of Superman, maybe it is a box of Bruce Wayne's used underwear.

If Superman didn't know Batman's secret identity, he does know. Other than that idk, maybe Batman placed a bomb in himself lol

@cameron83: I was thinking it's Thomas Wayne's letter too. A lot of people are taking this "Stop Batman" thing to literally. You don't have to kill him to stop him.