Cyburn writes : "Nintendo were generous enough to give those who registered a Nintendo Network ID on a 3DS between 10/12/13 and 31/1/14 a copy of Super Mario Bros DX for the Game Boy Color game in it’s 3DS incarnation for free. The codes arrive in the recipient’s email box, and they started being received on 14/2/14. Those who don’t have a code yet ; fret not! They are still in the process of being sent out (indeed up until the 28th February).
Super Mario Bros DX includes the NES games ‘Super Mario Bros’ and ‘Super Mario Bros : The Lost Levels’. Offering GBC adoptees bang for their buck, it introduced extra modes, save points and an over-world map".