First digit in s/n is last digit in year made. Serial number starting with "7" is a 1987, or 1997 or 2007. Telling the difference in the decade is from other attributes of the mower. I know '70s and '80s but not much beyond that. The series number will denote how new also. Series 0,1,2,4 and 5 are from like '79 thru '89. '90 I think started series 6.