protwin: Your model 57 serial number N77501 dates to 1980.
The 57-1 was manufactured between 1982 & 1988. The serial number prefix AJJ should have been used between July 1985 & November 1985. the serial number should only have four digiets though: AJJxxxx.
The serial number is sometimes stamped under the crane with the model number, there are often 'assembly' or parts numbers to confuse things. The Serial number is always stamped on the bottom of the grip frame, you will need to remove the grips to see this. If it is different to AJJxxxxx get back to us and one of us will look it up for you.