This is the info I found on the internet about sizes but there's no mention of an 18":
With the growing popularity of the original model, the AutoLock-21 (cat. #9020), we have decided to expand the line by adding two new models to the AutoLock family of batons, the AutoLock-22 with Power Safety Tip (cat. #9030) and the AutoLock-26 (cat. #9021).
The AutoLock-22 combines the advantages of the AutoLock with the advantages of our Officer Series Batons. It consists of the AutoLock-21 with the large, hard rubber Power Safety Tip that was first introduced with the Officer Series. This not only increases the length of the baton but also the weight and feel of the baton, making it more effective when performing strikes.
The AutoLock-26 is similar to the original AutoLock with the exception of its length. In response to the overwhelming demand for a longer AutoLock we now offer it in a 26-inch length.
http://www.police-batons.com/mlp/AutoLock/index.html