Nagatoro got the idea for the nipple guessing game from Love Slave

Chapter 12, Nagatoro Wiki

» Kono Subarashii Sekai ni Shukufuku wo!

Komi-san's Accidental Written Ownership - Chapter 10 - namichwan57 - 古見さんはコミュ症です。, Komi-san wa Komyushou Desu

Don't Toy With Me, Miss Nagatoro, Vol. 2 by nanashi

uesaka sumire – RABUJOI – An Anime Blog

Ijiranaide, Nagatoro-san - Episode 3 discussion : r/anime

flustered – RABUJOI – An Anime Blog

Nagatoro got the idea for the nipple-guessing game from the Shoujo Love Slave. : r/nagatoro

Kono Subarashii Sekai ni Shukufuku Wo! 2 – 04 - Random Curiosity

THELOVEKING (u/THELOVEKING) - Reddit

DISC] Ijiranaide, Nagatoro-san - Ch.54 : r/manga