go to the bank and ask them what legal documents they need to open box. They will probably require you to go to court and get Letters of Probate appointing you executor.
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Check the "Probate" box.
Read Probate Code Section 331 to see who to get into the safe deposit box, and what you can do once you have access.
If you don't know which bank it is, you've got to start there - figuring out the bank.
It's probably a bank that your father used for other accounts, although perhaps not. If you check the local banks that he used and the box is not with them, try some other banks near your father's house.