Extra chapter?

I'm trying to understand the chapter count. I'm seeing 77 chapters on ComicWalker, for instance - what about the extra chapter that the blurb here mentions and that is reflected in the character count here? Is it just a difference in how they're counted and it's actually bundled in one of the main 77 on ComicWalker, and if so, in which? Or, is it only on some external site, like one of the author's social media or something, and if so, where? I may have messed up but on the author's Pixiv by looking at the related tag all I could find were this image and this, which also has a bunch of unrelated stuff.