Recently purchased a 686-1. Looking on the heel of the butt, there is a serial number that consists of three letters followed by 4 numbers. This same number is also under the barrel, when I swing the cylinder out, on the frame where the crane contacts the frame.
However, I find another "serial" number.... This is 5 digits, all numbers. It is located in two places as well. First one is on the crane? yoke? in front of the cylinder. Second place is on the left hand side of the bottom of the butt.
Which of these is the serial number?