This Guest Post is by Mario McKenna of the Okinawa Karatedo Kitsilano Dojo in Vancouver, British Columbia, Canada. Mario is an instructor of Tou'on-ryu Karatedo and Ryukyu Kobudo. He is the English translator of Kobo Jizai Goshinjutsu Karate Kenpo (Kenwa Mabuni, 1934) and Seipai no Kenkyu Kobo Jizai Karate Kenpo (Kenwa Mabuni, 1934). His excellent article Okinawa Kata Classification: An Historical Overview appears at the Hawaii Karate Seinenkai website. Mario also has a new Karate and Kobudo Blog.
The sho designation does not indicate a shorter or simpler kata, although some people mistakenly think so. Indeed many times the dai version is less complex and taught earlier than the sho version. IMHO, it is simply a way to differentiate between two versions and/or lineages of the same kata. This is a guess on my part, but this designation probably began in the 1920’s and 1930's with the writings of Funakoshi, Motobu and Funakoshi while popularizing karate on the Japanese mainland.
If we look at Ryukyu Toudi Kempo (1922) by Funakoshi, you can see him introducing variants for Passai using the sho and dai suffix. Jumping ahead to the early 1930's you find Motobu Choki also using the sho / dai suffix when discussing kata in his Watashi no Toudi. Finally, in the mid 1930's you encounter the use of the sho / dai suffix in the works of Mabuni and Nakasone.
Although the designation of sho / dai may have existed earlier, I think it was these two gentlemen really popularized it. The sho / dai distinction is not really all that surprising given the penchant for Japanese to organize things to almost anal retentive levels. I am not surprised that these early authors used the sho / dai designation to possibly help their Japanese students differentiate between the two.