Possible Clues:

• Chart that might have a big "X" on it
• On which X marks the spot (... first 2 letters + last 2)
• It shows an X
• Prop in 'Raiders of the Lost Ark'
• Hunting item associated with one of this puzzle's special letters
• Instructions for a chest examination?
• Hunting guide of a sort
• Pirate's guide that's hinted at phonetically by the starts of 17-, 29- and 44-Across
• What this puzzle is