The first three or four numbers in the serial number will tell when the alternator was made. For instance, if your PennTex Alternator Serial Number starts with 105, the "1" stands for January and the "05" stands for 2005 so the alternator was made in January of 2005. If the serial number starts with 1005, the "10" stands for October and the "05" stands for 2005 so it was made in October of 2005. You would look up your serial number on the 2005 Serial Number List under January or October to find the alternator model. For numbers not listed here, call us at 877-590-7366 and we can match your serial number with a model number. Thanks for choosing Penntex.