I found the same system mentioned in the 1st comment of this blog article (“ハンディは９路盤は４段級差１子 […] 13路盤は２段級差１子”). The commenter apparently is a highschool/Go teacher.
Almost identical system, shifted by 0.5 kyu/dan difference:
Different system:
I haven’t been able to find description on the current system in use but I think it may be helpful to add it in the documentation or FAQ.