It tells you the version and operating system on which the project was created, in your case it says [6.5.1 (Windows)].
AE CS5 can open files from AE Version 6 and AE CS4 can open project files from AE version 5. Maybe try opening your AE 6.5.1 project with After Effects CS5 and saving it from there. For information on using older AE project files check out this thread.
I agree. This should be made more clear, especially since there is no quick and obvious way to find out such "minor" stuff. It's not covered in teh "What's new" section nor on Kopriva's blog post, but rather buried in the help.