CHICAGO (CBS) – As the Big Mac turns 50, McDonald’s is offering a birthday gift to fans of the iconic fast food sandwich.

Starting Thursday, Aug. 2, diners can get a “MacCoin” with the purchase of a Big Mac. That limited edition coin will be good for a free Big Mac beginning on Friday.

McDonald’s is distributing 6.2 million MacCoins in more than 50 countries. There are five versions of the coin, each featuring art from a different decade.

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The coins are redeemable until the end of the year.