The SMS Converter II will fit on either model of MD. The original converter will only fit the Mega Drive 1, unless you saw the tab off the back of the converter.
If you have a Mega Drive 1, you should go for the original Master System Converter/Power Base Converter - it's more common, looks neater, and has a card slot so you can use card games or the 3D glasses. The pause button is more conveniently placed as well.
Mike