Alice/Julia
Rupert Chatwin was the one who found the key and brought it to Brakebills in the late forties to give to his friend, Lance Morrison. Um, I say friend but, they were lovers? Rupert gave the key to his friend to reveal the truth of their feelings for one another, I guess.
Chatwin said the nature of the key was to reveal the truth of things. The key remained in Morrison's family and we were able to track down its current location.