How do you get the master key for boss 7?

A master key is an original operating key and operates all locks on a vehicle. Sometimes there are two keys, a master and a valet. The valet key is used when you drop off a car for someone else to...

It was published in 1945.

Master keys open all rooms in a building. Grand Master keys open all rooms in several buildings.

Just like any other key ... except that a master key will open more than one lock.

After you jump off the boat, there will be a door on your right with three more rooms in it. Find the one with the wooden spiked walls and use Din's fire. Then you'll see the chest.