@Sollertia_
I just put 7.1 because it came as an omake at the end of the volume. As a prologue, it should be chapter 0 but then new readers to the title might get as little confused. Space-wise, it might best fit as 1.1, kind of as a short flashback after the first chapter, but then as a flashback, it fits anywhere.
Also, I found a 2 page short the author did to celebrate tankoubon release that didn't make it to the tankoubon itself, and I'll get you that sometime.