Help - New Type Coin Or Error? Mexico 50 Centavos 1956

I have rolls of BU original 1956 50 centavos coins, and dozens of many other dates. All show the same characteristics. Recently I found one that is a different color, a more white coin. Both my others and the new one weigh exactly the same - 14 grams. I speculated that perhaps someone had dipped it, but in over a year there has been no change in its appearance. Today I looked at the new coin, and noticed the five had a shorter top to the number, so I took pictures and tried to measure the difference. I needn't have bothered. Check the pictures. The 9 is even more different. The circle of the 9 on the new coin is square. You'll notice the new coin is almost white. Take a look at the pictures, compare with some of your own, and please help me decide what this . You will notice differences in the reeded edges.