DMM/FANZA: https://www.dmm.co.jp/dc/doujin/-/detail/=/cid=d_227915/ DLsite: https://www.dlsite.com/maniax/work/=/product_id/RJ383240.html Another extra doujinshi chapter to Hero no Himitsu (ヒーローの秘密) by Imamura Youkou (陽子今村) (yes, the romaji of the author's name is wrong on this cover), which can be found translated into English on MangaDex here: https://mangadex.org/title/b4a21b67-22e7-4482-add5-8cd9451bedfe/hero-no-himitsu When I have time and money, I'll get this translated into English like the rest of the series.
On page 22 you can see a reference to one of the author's other works and I tried to find it again but the only version I can find has the pages out of order and some of the pages are even messed up with like the bottom half of one page being pasted over by the top half of another page? What a shame.