In Brazil Sega Mega Drive was also renamed the Mega Drive "II" too coincide with the Sonic game before the resized USA, JPN, PAL system versions. So are version 2 is there version 3. All in all there are 5 versions, USA Sega Genesis, Japan Sega Mega Drive, Korea Samsung Mega Drive, Brazil (TECTOY) "Sega" Mega Drive and (PAL) Europe Sega Mega Drive....
They all look different so they can be counted as different systems.