Responses

Thanks V-chan. Umm.. Not really extra, it's another story.
Yasunori & Seimei are real figures in history. More info here..
http://www.mangago.zone/read-manga/seinenki_abenoseimei_ibun_reimei_no_hana/mf/v01/c001/20/
So some mangaka made their own manga, based on this fact, which were quite different story, depended on each mangaka's imagination.
I found a Extra Chapter (although its written by a totally different mangaka):
http://www.mangago.zone/read-manga/anohito_to_nitenai_kuchibiru/mf/v01/c004/