Hi Joe,
your question has to be more precisely phrased. DM stands for Deutsche Munition (German Munition). So every military munition, cartridge, fuze, grenade, bomb has a DM No. To identify an item you have to name its family, like : Leuchtpatrone (signalcartridge), its speciality,like : Einzelstern,gelb (single star yellow) and its DM No., which only denotes the model modification. There may be many different items with the same DM No.66. DM 66 only says that this is the 66th modification of a certain munitions item. In your case if its a flare fired from a pistol it most probably will be a 26,5mm cartridge, but what family. Signal, Reconnosance, Light, Sound, Colour, No. of stars, Parachute. To answer your question more info is needed.
Regards,
Bellifortis.