three numbers in the head number: first number is the number of rows second number is the model of head third number is row width (3=30",4=36_38") a 693 would be a six row, 90 series, 30" head. a 444 would be a four row, 40 series, 36_38" head. the second number will be either a 4 or a 9 the 4 series was built for many years and the 9 series is the most recent model using poly rather than the metal snouts. very good heads with only minor improvements made over the years. I hope my explanation is clear and accurate.-mike