Numbers are cast into the head below the second intake port back from the front of the head. A sequence of numbers will include a single number such as 14 or very rare 15 or 17 or 22. Try to get a 22 head as this is the most recent upgrade. There might be some heads with numbers between 17 and 22 also. Doubt you could get a new 14, 15, but there might be some new 17's around, but doubt that too! BB