It's the fifth number on the serial number on back of the radio. The one that has the Z in it. The serial number and the information in the radio I think it's the 7th number but it's definitely 5th number on the back of the set. Mine is a 0.
By the way the 8 that follows the date means that was manufactured in Vietnam and the last numbers after the zeros, it's the number machine made that year and it starts over with each New Year.