Thanks for all your suggestions, I weighed it and it was about .008 of an oz. lighter. It would not twist apart but I was able to very carefully pry it apart. The outside diameter of the coin was the standard 19mm, it has a well machine cavity 13.5mm in diameter and 2.5mm deep total between both halves, a little less than two dime's thickness. The obverse side is the female 2mm thick and it has a 1.25mm cavity in its half. The reverse side had a inset male shoulder with a total height of 2mm with the coin's outer diameter lip 1.25mm thick, its cavity was 13.5mm wide and 1.25mm deep. It has a small folded old paper inside. The coin was a family heirloom I inherited, probable came from great-great grandfather who was a high-end male hat merchant in Philadelphia who collected coins and signatures. The machining is very precise, even, clean and professional. The coin has been in my immediate family for at least 71 years and no one in the family was a machinist. What was interesting, the coin was in an old slide type holder that had be stapled shut but pried open and showed evidence of being looked at many times but the damage to the holder. None of the other coins showed that same intrusion, so the coin must had raised a lot of questions. After finding the cavity last night I might have vague recollections of knowing that as a child. Regarding what it might be is still the question. It is to hard and small to get apart to be for medicine or a magician trick, any message would have to be very small.