Key to a safe deposit box and an address of the bank is more than enough to fulfill that requirement. At my bank, they come with two keys. Put one in the box and keep the other, and check the safe deposit box periodically.
I don't have the time to dig this up, but banking law forbids unknown parties to access safe deposit box in major banks. Sure you can give the second key to anyone you want but when they showed up at the bank, they will have to show their ID that matches name on the list of people you gave permission to access the box (at least that's how it is at Bank of America, Chase and Wells Fargo - I don't have boxes at other banks). This is part of KYC (know your customer) routine for banking institutions.
All he needed to do is leave a will-type of letter in this chest and directs you to bring this letter to his lawyer and that will do it.
You're right, I actually asked a banker after I wrote that and she said you'd need to show your ID that matches the name on the account. Sorry, too many bank heist movies I guess.
At my credit union they've moved to using thumbprints on a reader for regular visitors so the staff doesn't have to go through checking signatures, etc.
I'm executor for my aunt. She has a safe deposit box at a bank to which I have her key but I'm not on the list of people allowed access. They would have allowed me to open the box and remove a copy of the will if that was the only place it existed (that was also the situation with my dad) but otherwise I would not be allowed access.
Interestingly the agreements I've signed specifically disclaim any responsibility for gold, silver, cash and similar items.
