The bronze key is an item that players can steal from H.A.M. Guards in the storerooms (lower level) of the H.A.M. Hideout.
It is used to open the small chest in the south-west storeroom. To enter the room, the player needs to pick the lock on the door while the guard is facing away. If the guard sees the player picking the lock, the guard will attack them.
The chest gives a reward of coins or 1-3 pieces of random jewellery and cut gems. It is possible to not receive anything from the chest.
After using the key to unlock the chest, the player receives a message in the chatbox saying: "Your bronze key breaks in the lock." and the key disappears from the inventory.
The location of the chest.
For an exhaustive list of all known sources for this item, see here
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