Antique Shakudo Japnese Cigarette Case

This is a beautiful Shakudo Cigarette Case which we believe belonged to my partners Grandfather. I have been trying to do some research but struggling. I cannot find out what the symbol on the first photo means it is on the inside of the case. On the outside of the case it has a mountain and Japanse sailing boats in what looks like gold and silver and on the back of the case are two small birds one in what looks like gold and one that looks like Silver. Does anyone know how I can tell if this is genuine Shakudo someone mentioned something to do with a magnet but not sure how that should tell me what it is made of? I am pretty sure it is Shakudo as it is very old.