Key to the Bunker
The key belonged to the Men of Letters and was kept in a box which was decorated with their crest - the Aquarian Star. Inside the box is "the key to every object, scroll, and spell ever collected, for thousands of years, under one roof". Basically, the Men of Letters had stored, in one location, all the supernatural knowledge that they had ever possessed. If the Box were to be hunted down by any "evil being" (demons, monsters, etc...) it would be safe when stored in the area where they had placed all their scrolls and secrets because the area itself is branded with symbols to ward off every kind of evil ever created.
In 1958 Abaddon, one of the Knights of Hell, was sent by party or parties unknown to kill the members of the group who were gathered in Normal, Illinois. They were gathered for the initiation of Henry Winchester and Josie Sands.
Abaddon killed David Ackers and Ted Bowen, and blinded Larry Ganem. After Henry bursts through to find out what was happening, Larry crawls up to Henry and entrusts him with a box which contains the key to where all the knowledge the Men of Letters has collected is stored. She then went on to burn down their quarters before following Henry through the portal created by his blood spell. Sam visits Larry Ganem, now very old and the only survivor of the incident, to find out more about the box and its contents. After Larry explains everything, Abaddon, who has possessed Larry's wife and was listening to the entire conversation, kills Larry and captures Sam. Abaddon then calls Dean to demand a trade: Sam for Henry & the key. During the trade, Henry shoots Abaddon in the head with a bullet that had a devil's trap carved into it. This immobilizes Abaddon, and Dean then cuts her up into pieces. After Henry's death, Sam and Dean decide to find out if the Bunker is still intact.
Sam and Dean follow the coordinates provided by Larry, and find an underground Bunker, which they are able to enter using the key.
Dean hands Kevin the key to the Men of Letters Bunker where he can be safely protected.