Interesting, in the family style of Shorin-ryu, Matsumura Seito, the original kusanku kata was practiced with the jiffa once one was proficient at the kata. In the 1970's in an interview, Hohan Soken, the inheritor of the system, described doing the kata with the hairpins as taught by his uncle and teacher, Nabe Matsumura (Bushi Matsumura's grandson). And as mentioned before, Bushi Matsumura's other student, Itosu, later broke the kusanku kata into Sho and Dai. He probably was also taught to train the kata with the jiffa.
Thank you, Walt. I think you just solved an old mystery for me. There is a move in Matsubayashi Kusanku (and Pinan Yondan) where a "punch" is thrown from the opposite hip almost as one would throw a backfist, except that in the Matsubayashi version (although not all Shorin Ryu versions) the fist is horizontal (pronated). It is concurrent, more or less, with a front kick. It does work as a backfist, but that is not the correct Matsubayashi form, and as a "punch" or hammerfist technique I have always considered it relatively weak. However, as a jiffa strike it makes perfect sense!