Musubi is basically a rice ball. In Hawaii, Japanese immigrants who mainly worked in the plantations added a musubi to their bento (lunch) box as they went to work in the fields. The rice was usually left over from the night before, and was lightly salted and shaped into a ball, or a triangle. Some musubi were stuffed with pickled vegetables (tsukemono) and/or wrapped with nori (seaweed)
After World War II the Spam Musubi took form, as spam was used a lot by the troops. The spam was grilled, then placed on a bed of rice, and wrapped with a small piece of nori.
Pre-711, Spam Musubi was usually made by mom and packed for lunch, or sold by the counter at a local corner store. Once 711 entered the Hawaii market Spam Musubi took a life of its own, causing the Spam Musubi revolution. 711 sold a lot of it, and major grocery stores started to pick it up.
711 was the catalyst for the Spam Musubi evolution since there were now more places to buy it, they had to create new flavors such as spam and egg, teriyaki spam, hot dog, chicken katsu (cutlet), tonkatsu (pork cutlet), and more.
When you come to Hawaii be sure you try a Spam Musubi