Triple nickel means that they plated the bumpers in three nickel plating steps. If I remember from when I worked at a plating shop correctly the triple nickle plating process is as follows: copper strike, dull nickel, bright nickel, porous nickel followed by chrome. My guess is if it is called a double nickle it is just copper strike, followed by dull nickel, bright nickel and then chrome. The porous nickel is for corrosion protection.