This is obviously just a generic UK plug -> USB mains adapter connected to a USB cable that fits the 3DS' input port. Both US (100-120V) and UK (230-240V) official chargers output at DC4.6V, 900mA. This outputs at DC5.0V, 500mA.
Don't buy it, don't use it. Waste of time and money. You might say "oh it works just fine" initially, but you shouldn't use the wrong voltage and amperage when charging your devices, end of story.