There are three distinct I/O Controllers found in the Master System. They are classified by numbers 315-5297, 315-5216 and 315-5237.
The first is found in Japanese Master System consoles [1]. Broadly, the second is found in export Master System I consoles and the third in export Master System II consoles, but the opposite is found in a small number of cases. Nevertheless, the terms "Japanese SMS", "Export SMS1" and "Export SMS2" are used to distinguish between the three I/O Controllers because the terms are more memorable.