It means Singer set aside 50,000 serial numbers to use for the Model 66 at that time, it really does not say how many they actually produced, or when -
I think it's relatively safe to say your machine was made between 2/3/1912 and 12/18/1912 when Singer next allocated serial numbers for model 66. But you can almost guarentee it was NOT made on 2/3/1912, as it's reasonable to assume that Singer allocated numbers before they actually needed to use them.