Here is a history of Seisan by Joe Swift.
Seisan (Hangetsu)
Meaning 13, some people refer to this kata as 13 hands, 13 fists, or 13 steps. Customarily taught in both Tomari and Naha (towns on Okinawa), this kata, following the tradition of Chotoku Kyan, is the first kata the Isshinryu karate student learns. A move from Seisan is here demonstrated by Funakoshi in his 1935 book.
It is unclear exactly what the number 13 represents. Some think it was the number of techniques in the original kata; some think it represents 13 different types of "power" or "energy" found in the kata; some think it represents the number of different application principles; some think it represents defending against 13 specific attacks; and some think that it is the number if imaginary opponents one faces while performing the kata. Out of all these theories, this author must disagree with the last, as it is highly unrealistic that kata teaches one to handle such situations. On the contrary, kata was designed to teach the principles needed to survive more common self-defense situations, rather than a long, drawn out battle against several opponents (Iwai, 1992).
Akio Kinjo, the noted Okinawan karate researcher and teacher who has traveled to China, Hong Kong and Taiwan well over 100 times to train and research the roots of Okinawan martial arts, maintains that this kata originally had 13 techniques, but due to a long process of evolution, more techniques were added to it (Kinjo, 1999). He also maintains that the Okinawan Seisan kata derives from Yong Chun White Crane boxing from Fujian Province in Southern China.
It is unsure who brought this kata to Okinawa, but we do know that in 1867, Seisho Aragaki (1840-1920), a master of the Chinese-based fighting traditions (Toudi) demonstrated this kata (among others) in front of the last Sappushi (Chinese Imperial envoys), Xin Zhao (Tomoyori, 1992; McCarthy, 1995, 1999).
The main lineages that include Seisan include those passed down from Kosaku Matsumora /Kodatsu Iha /Chojo Oshiro, Chotoku Kyan, Seisho Aragaki, Kanryo Higaonna, Kanbun Uechi, and Norisato Nakaima, among others. Shimabuku learned this kata from Kyan. Both the Kyan and the Shimabuku versions of this kata strongly resemble the "Matsumura no Seisan" used in some sects of ****o-ryu (see Sakagami, 1978).
Noted senior Okinawan karate authority Hiroshi Kinjo (b. 1919) states that there is no evidence of a Seisan kata being passed down in the "Shuri" lineages of Sokon Matsumura and Anko Itosu, and that the familiar "Shuri" lineage Seisan versions such as the Hangetsu of Shotokan and the Seisan of Kyan lineage systems, should be referred to as Tomari Seisan. His reasoning is that the so-called Oshiro Seisan as presented in the 1930 "Kenpo Gaisetsu" by Nisaburo Miki and Mizuho Takada was actually passed down from Kosaku Matsumora to Kodatsu Iha to Kinjo's own teacher Chojo Oshiro of Yamaneryu Bojutsu fame.
Kinjo believes that Funakoshi, being a school teacher in Tomari (the small port town near Shuri, Okinawa's capital city), may have learned same from Iha, and that as much of Kyan's tutelage seems to have come from Tomari-based masters like Kosaku Matsumora and Kokan Oyadomari, the Kyan version could also likely be traced to Tomari (Kinjo, 2001). This is in direct contrast to many writings on the lineage of Kyan's Seisan (see, for example, Bishop, 1999).
The "Master Seisan Theory," which claims that the kata was brought from China to Okinawa by a Chinese martial artist named Seishan (or Seisan), is uncorroborated myth at best, probably propagated by well-meaning, but not-so-well-researched instructors. This legend cannot be found in any of the literature coming out of Okinawa or Japan.
Martial Art Style Tang Soo Do
In this case...13 does refer to the 8 directions and the 5 states of matter (5 elements). To understand a good direction to turn is Taoist alchemy which originated the theory and application. The concept of 5 elements and 8 directions has played a part in many martial arts, but best known for its association to Tai Chi, Ba Gua and the geomancer's art Feng Shui. It has also been applied to medicine, psychology and war strategy. I personally believe Ship Sam came to us via Okinawa - Japan - Korea...but has its origin in Tai Chi Chuan (probably the Chen version). Too many of the moves in the form are close matches to movements in Tai Chi to be coincidence in my mind.
JH