Menko were Taboo
It is often difficult for us to get information on menko. That is not only because menko is an old toy for us. There is another interesting reason, too. Actually menko has been considered a bit of a "morally undesirable" toy because it is with a "win or lose" or "gambling" aspect. In the old Showa age, many schools banned children from bringing menko into schools, as it was morally unacceptable bringing such a win/lose thing.
Even in Edo era (17th century)( in this age, menko was made of clay, not paper), the samurai government (Bakufu) declared that it was desirable to have children stay away from menko games as menko would facilitate a speculative or gambling spirit.
So, there were not many advertisements and sales promotion for menko cards unlike other "educationally good" toys. That makes it hard for us to get info on menko.
And, also, such a "morally undesirable" "underground" aspect was exactly why menko were such exciting toys for children! The "hard to get info" aspect is now exactly why I find it interesting to try and find sumo menko info!
Menko of the past are similar to the POG craze that swept through the US in the early 1990's. Even POGs were banned in many schools throughout the US because of it's gambling aspect.