Given the metal insert for holding the object(s) it is not logical that it is a map holder.
In time of War who would want to fool around rolling a map up, putting it in the holder, putting the holder in the tube and putting the cap on the tube.
Or the reverse of ..... to get the map out.
Not to mention the fact that carrying a "banging clanging" bulky object such as that is ....... would make for an inconvienence.
Especially when a leather pouch or tube would have worked just fine, more practical and more easily produced.
HA, ... rideing horses is somewhat different than rideing Jeeps or Humvees.
But whatta I know?