I wouldn't buy a narrow band a/f meter for any valid testing and tuning. Save your money and get a wide band like the innovative LM1 from our host airspeed or theres the techedge DIY wideband and if ya want you can buy assembled ones from them.
Narrow band a/f meters will not give you any valid/useful readings that will provide you any useful info. Just blinking lights. No disrespect but you'll never see serious car enthusiasts using a narrowband for race purposes.