Need Help Identifying Alpha I Trim Cylinder

[Rock Hall]

I have a 1989 Mercruiser Alpha I Gen I Stern Drive. I need to replace the port side trim cylinder. It seems that there are 2 different length tubes. I did a measurement (under water) over the weekend. I came up with 16 1/4 to 16 1/2 inches. The sales guy says that most 89s are the shorter cylinder. The boat is in the water and is kind of a hassle to get out of the water. I can not get the casting number from the tube due to the rough condition of both tubes, especially under water. Does anyone know any hints or other differences on these tubes I could look for. My measurement is indicating that it is the longer tube contrary to the salesmens experience.
Thanks
Mark