Strongbox
A chest with a secret content.
Captain's strongbox
A strongbox, also known as a Captain's strongbox, is a lockable chest. But extra strong and with better locking abilities than your everyday travel trunk.
The captain of a ship would use such a strongbox to store valuables, such as cash and documents.
Part of the ship, part of the crew
Sometimes the strongbox was owned by the captain and went with the captain if they went to another ship. Then maybe they would also store personal items in there. Other times, the strongbox was part of the ship's inventory. And it could well be built into the ship's structure. Even hidden. Nowadays those things are called safe or vault.
Design
Traditional strongbox came in many sizes, from small ones that could contain several letters and maybe a small purse of coins, to full own treasure chests laden with gold and jewellery. In the past they were made of wood and reïnforced with metal straps and corners. Nowadays they are fully steel. They can be locked with key's sometimes the locks are hidden by puzzle boxes and you need to push a certain rivet to open a latch to find the keyhole. Or they have a simple padlock. Modern boxes may be equipped with a number lock.
Another telling feature is that the strongbox is often designed so that it can be secured to the ship's structure. sometimes with handles or rings on the side for lashings, sometimes there are accomodations for bolts to go striaght trough the bottom.

Rose always had to make sure the strongbox of the Sunset Dawn was closed, because Roger loved to crawl inside of it. And when the ship rolled the lid could fall shut and lock. Turning poor roger from a Ship's cat into Schrödingers cat. But to be fair, Roger's share of any treasure was kept in the strongbox, so he had every right to go check it.
Desguise
The best defence against burglars is making sure that the strongbox doesn't look like a strongbox, but that it mimics another thing. Or the actual valuables are not in the strongbox but somewhere more mundane, and the chest just contains a small token that could make a would be thief content.
Strongboxes on the different pirate vessels:
Narwhal
The Narwhal doesn't have a strongbox like you'd think of it, but Peter has a lock on the drawer under his bunk. Where he keeps all kinds of paperwork and a small wallet of cash. The original papers from the boat, some crew members their real documents, while they have fake ones to show to any nosey officials.
Sunset Dawn
The Sunset Dawn Dawn didn't come with an interior or furniture. The first crew of the Antilope just hung up some sheets for privacy and used their own suitcases for storage. When Rose became captain she had Arne Anderson who was good with wood, make a strongbox. Crafted from Liveoak and black steel, with a false bottom insert. Like Peter before her, it mostly stored documents, such as their Letter of marque.
Red Sunrise
The strongbox from the Sunset Dawn actually survived the sinking, but when the Red Sunrise was built, the box was brought up from the wreckage and was among the items that got reused on the new Pirate ship.
Medicine chest
Another thing that can be stored in the strongbox are strong or addictive medicines if the Sickbay doesn't have a good locking option.
That example strongbox you found is gorgeous. I love all the different appearances and uses different ships could have for a strongbox. I never thought about storing important documents there.
Explore Etrea | March of 31 Tales
Wikimedia is actually a good source for nice pictures for this world! Important documents, or just the real documents, while you present false cargo papers to a harbour master to avoid some taxes...