Possible Clues:

• Is that the mail the sentry is sending?
• Letters to be sent from the sentry?
• Letters to be sent from remote station?
• One must be on the lookout for one in mail
• Rejected job in remote settlement
• Collect the letters sent from the sentries
• Letters awaiting despatch to a colonial settlement
• How letters go away from the sentry
• Does one watch for the letters leaving?
• How mail gets collected from the sentries