Love Letter Soudatsusen is an original short story written by Kamachi Kazuma exclusively for Haimura Kiyotaka's artbook rainbow spectrum: colors.

The short story primarily focuses on Kumokawa Seria's mysterious love letter that is to be delivered to an absent Kamijou Touma, and the hilarious antics several characters get into in order for them to attain the letter.

(Source: Toaru Majutsu no Index Wikia)