What's the part number of the original plugs that were installed? Like the previous poster stated, just because the original plugs in there seemed to work, doesn't mean they were correct.
You stated that the thread length was the same as the mis-matched plugs that you installed. Like Don pointed out, the witness marks from the carbon on the picture of the plug you posted show that the plug is most likely too long for your cylinder head. Your head needs the correct length spark plug installed. If the threads are too long, then you are placing the business end of the plug too far into the combustion chamber, and your also putting sharp edges into the combustion chambers (the threads) that will promote hot spots and pre-combustion.
Theoretically, if you could look at the other side of the cylinder head with the plug installed, the base of the plug should be flush with the cylinder head with only the electrode and ground strap being exposed in the combustion chamber.
