Just purchased a 66-16(?) which had a 66-6 manual with it. The serial number in the lower right corner of the machine does not match for a 66-16. It says it is a 221-1. The serial number is AG 853182. I'm not sure about the A. It is faint on the plaque. From pictures I have compared it to, it is a 66-16. How can a serial number not match a model? Please help with some answers.