Magically identifying an iron safe (if you manage to pick one up, that is) allows you to open it, because you identify the combination.

The Master Key of Thievery can be used to lock and unlock iron safes despite them not having a keyhole.

Iron safe additions:

• When you crack an iron safe with a stethoscope, it sets an internal flag that indicates that you know the combination. (Opening it with opening magic won’t do this.) You can open any safe you have already cracked once, without needing to crack it again.
• You can manually shut an iron safe with the close command.
• Sometimes, the Oracle will tell you the combination of the next iron safe you will try to open as a minor consultation. The next time you open a safe that you don’t know the combination of, you get a message “You enter the combination the Oracle told you: X-Y-Z”, and the safe opens in one turn.
• If you somehow manage to pick up and read an iron safe, it says “Croesus Safe & Lock Company, Fort Ludios, Yendor”.