open the hood and look on the serial number tag for the UNIT number it will have a number like " 5A15C " First number is the year ie 65,75,85,95
Letter is the month A=Jan B=Feb etc note they skip the letter I because it looked too much like a one
second set of number is the day of the month
last letter is the work shift A= day B= evening
C= night shift
if the tag under the hood is gone, this same number is on the trans housing along with the serial number
good luck !