Re: Source of the nameing conventions.
It would make a nice story if ikkyo, nikyo, sankyo, yonkyo were contrived as distillations of the corresponding series of kata in the Hiden Mokuroku.
I.e. ikkyo captures the core principal of the ikkajo series, nikyo captures the core principle of the nikkajo series, etc.
This is something I have heard before but it is almost too tidy to hope for.