The symbol of the Men of Letters was worn by Mary Winchester (Sam and Dean's mom) as a charm on her bracelet in 4x03 : In the Beginning (2008).
Jensen Ackles (Dean) was really sleeping in this episode. The director had Jared Padalecki (Sam) wake him up. They use his real reaction in the final cut.
The Men of Letters Bunker is located in Lebanon, KS, which is the geographic the center of the lower contiguous 48 states of the United States of America.
The title of this episode is from the 1931 song of the same name that the character Sam sings in Rick's bar in the film classic Casablanca (1942).