The Mystery Deepens - Merging 2 1950s Sumo Wreslting Menko Sets

My quest to catalogue every sumo menko and card produced and publish my findings has been driving me over the past ~20 years.  These days discovering news sets is few and far between, but this past month I was able to solve a part of a "mystery" that has been bugging me for a while.  The BC598: 1959 Trophy 6 set always stood out as a loner.  These five trophy BC-series menko cards seemed out of place because they only depicted the trophy hardware, never wrestlers.  But I could never pinpoint where they belonged so I gave them their own catalogue number and set name until I could find out.  However, I recently discovered an uncut pair of BC5710: 1957 Yamakatsu Gyoji 5 menko that had both a BC598 and BC5710 on it.  Mystery solved or so it seems.  For some reason I never caught on that on the back of the trophy cards, they had the years associated with the last 8 winners of the trophy that was depicted on the front.  When do the years end?  In the middle of 1958.  So I had the wrong year on both of these sets.  So now I will merge both of these sets and recatalogue them being from 1958, but the question remains, why did Yamakatsu issue these difficult-to-find trophy cards as well as give them a 6-digit Fighting number instead of 5 digits like the rest of their set counterparts?  Were they used for some other prize?  The mystery deepens....