This is a patch for Zen Cart Versions 1.5.5a and older ONLY. MasterCard has begun issuing credit card numbers starting with #2, which is not a part of Zen Cart coding. If you accept MasterCard and someone uses one of the newer card numbers beginning with #2, the order will not go through.
It is always best to have your store operating at the latest version of Zen Cart. However, if you just need a fix, we are offering it here.