I have had ten different people send me what they claimed was the only decipherment of C1. Each and everyone was different from the other and after checking each out both their decipherment and where they said the treasure was located, they were all wrong.
I also had two individuals that dowsed a map and sent me a copy of the topo-map, they too were wrong.
I do not believe C1 can be deciphered because the author proves in the story and in his own decipherment of C2 that the story is just made up.
It is just a story to sell like all the other dime novels of that time and era.