Discovering new menko sets is always a thrill.  Discovering new C-Series/Round menko sets is awesome and on a whole new level.  It is what keeps me going in the sumo card hobby.  I came across this set recently and knew it was extremely unique when I got it in hand.  Why?  First, it was printed in 1930 during a time when only a few cards/postcards were being printed of rikishi so it captures some obscure wrestlers.  Second, C-series menko were not popular so only a few of these types of sets were printed and even fewer survive today.  And lastly, this is the only C-series set that actually uses real photos of wrestlers instead of drawings.  This set is constructed by printing the images on paper, gluing that paper to thin cardboard, and then die cutting the menko out.  On some of my menko the paper and cardboard are separating as the 90-year old glue is starting to lose its grip.
You can see the glue separation as well as the layout of the menko on this miscut example:




As an added bonus, this set's checklist is the largest of the type coming in at 14 cards.  Each menko measures approximately 1.75"/4.5cm in diameter.

Here are the four colors shown with the Gyoji card - Grey, Purple, Pink, Blue: