"MAD" is the manufacturer's identification code for Mad River Canoe. "EX" is a model code for the Explorer. "584" is a serialized number. It might mean that the boat was the 584th Mad River Canoe made that year, or perhaps the 584th Explorer made, or perhaps something else. But it is the number specific to that hull and must be unique.
The "G686" at the end of the HIN indicates that the hull was laid up and certified with the US Coast Guard in July of 1986 to be available for sale in the 1986 model year. The "-K" at the end is not part of the actual HIN. MRC appended this to the end of the HIN on their Kevlar boats.
