From the Super Mario Wiki
Originally belonging to Harry T., the Storeroom Key is stolen during Chapter 4 when the Shy Guys invade Toad Town (this can be seen if Mario visits the shop at the beginning of Chapter 4). The key is then put in a treasure chest at the east end of the Blue Station area of the Shy Guy's Toy Box. After getting the key, Mario can return it to Harry T. or use it to open the door of the storeroom directly; in either case, Harry will allow him to take whatever he wants in the storeroom. Retrieving the key is necessary as Mario needs the Toy Train in the storeroom to go to the other areas of the toy box.
The Storeroom Key uses the same sprite as the Odd Key.
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