Back in 1941, right before World War II started (or at least between the U.S. and Japan), an unknown Japanese company produced this Math Back menko set in order to entice kids to buy them in hopes that they would not only play with them, but use them as a learning tool as well. There are numerous examples throughout the 1940s and 1950s of Japanese companies doing this. They are the precursors to flash cards it seems. These three menko that recently came up for auction are the only three of this set I have seen in my close to 20 years of collecting sumo menko making them extremely rare. This small lot features the great Yokozuna Futabayama (75/5=), the weak Yokozuna Minanogawa(27+5=), and the even weaker Yokozuna Akinoumi shown here as an Ozeki (50+4=). Amazingly, these menko survived the air raids and destruction of the city they resided in starting 79 years ago.
Take care and good luck with all the home schooling and online learning!!