Apache Tomcat 5.5.x supports the same Servlet and JSP Specification versions as Apache Tomcat 5.0.x.
There are significant changes in many areas under the hood, resulting in improved performance, stability, and total
cost of ownership. Please refer to the Apache Tomcat 5.5 Changelog for details.
So, from that I gather that there is no change in the specification in any of the 5.x versions. So it seems it would make sense to use the latest 5.x.?
That is the page I linked to in my post above - it doesn't actually say what spec that Tomcat 5.X implements. It mentions what every version of Tomcat other than 5.X implements - which I found strange.