OK. it is a treasure map. I have further questions.
it is important to know the common language of the time. is this Spanish? Turkish? or what? What country is this in? You don't need to give away precise location, but just general area.
I'd want to examine the stone with a magnifying glass to see all the details close up.
The line that goes around the center could be a trail that leads from where you found the stone to the cache site. Or it may outline something/s visible from the where the stone was buried. You need to know much about the area -- rivers, streams, mountains, hills, lakes, trails, roads, etc. Today, much valuable research can be done with Google Earth. You can become extremely familiar with the area using the virtual reality of google earth.
Also, you need to see if such symbols have been used in pictographs found elsewhere in the region and see what they are thought to mean in those places. In our case, there were books available which had lists of Spanish symbols and what they meant. That made our search easier.
Also, Who were these bandits? Were they educated? or simple peasants? What did they rob or steal and why? Why would they hide the loot in this area?
One of the most important questions to ask is: What would you do if you were them? Where you you hide it? How would you hide it and why? How would you mark up a treasure map given the circumstances?
You need to know these bandits like they were your best friends. You have an extremely valuable source-- the bandits son! Go over the story hundreds of times. You may be surprised when some little seemingly unimportant point may be the key to the whole thing. And don't be surprised when you suddenly see something you had seen, but overlooked, before. That happened to us several times.
These are some of the questions you need to ask yourself.