This is a fabulous treasure...NO DOUBT ABOUT IT. Texas Rangers have always been proud of the firearms and have been known to embellish these historical tools back to the beginning of Texas. I would carefully and cautiously say that don't be surprised if you find out that the revolver was engraved after it left the factory, and at that time it may have been re-nickeled or silver plated.
I would be very interested in the information provided by the Historical Archives, BUT it should be followed up with Local history and pictures of your grandfather if at all possible.
Thank You for sharing.