Hi, Jake 'The Cool Person' here.
The Ace Key opens the safe marked with an 'A'. It is on the other side of the plate where
the Ruby plate is on Easy and Normal.
To get there, go to the other side of the room (near the ruby plate), and push the box
(Keep going!). Go around, climb it, and use the key. On Easy, you get a Magnum Handgun and
Magnum Handgun rounds.