This is a lock I got from eBay - they were relatively cheap since they had no guard key, but I made an initial guard key with a file and some HPC flat stock. I bought some blanks and cut them with my Framon 2 machine, so now I have working locks. There are only two guard levers, so I don't think it's as secure as modern safe deposit locks. The operation is the same though - you use the guard key first, then the renter's key will work. When the key is turned back, the levers are moved so they rest on the stump - using the renter key resets the position of the guard levers so they drop down, and the guard key must be used again if the lock is to be opened.
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