|The following is based on Fallout: Brotherhood of Steel and has not been confirmed by canon sources.|
A blue flask for holding water, inscripted with number 13.
Found in the Mill Storage Area, the Vault 13 flask is marked as a "Quest Item" and can only be given to the Wasteland stranger in exchange for 100 bottle caps & 100 Experience Points. On receiving the flask the Wasteland stranger remarks on the condition, noting "the old vault colors haven't even faded that much".
Given the sentiment the Stranger expresses seeing it, it must be refering to a flask once used in Vault 13, the vault the Dweller orginally came from.