The Sanna 77 was a semiauto copy of the Czech Sa 25 submachine gun. It was first produced in Rhodesia by the GM Steel company for the Rhodesian military. In this form, it was the GM-15 and GM-16 (available as either civilian semiauto or military full auto), and was made without and licensing agreement. The Rhodesian military wanted more submachine guns, and a captured Czech example looked like a simple and effective weapon that would be easily manufactured.
It appears that when the tooling was made for the GM-15/16, a second set was also produced, and brought into South Africa. In 1978, the Sanna 77 was put on the market as a semiauto only version of the GM-15/16. It differed only in the stock, which was a single polymer part (instead of a the metal stock of the Czech original and the Rhodesian copy). A substantial number were made and sold by 1983, when production ceased. The remain relatively common in South Africa today, although they have a poor reputation for reliability and virtually all of the stocks have broken.