From the research I have done, it seems that Nero 126.96.36.199, Nero 188.8.131.52a, Nero 184.108.40.206c, and Nero 220.127.116.11d are all almost the same (including version number from “About”), differing only in a DVD codec. Their installers also differ in what they may install: Nero 18.104.22.168c and Nero 22.214.171.124d come bundled with the third-party Ask.com toolbar and The Weather Channel Desktop application. Installing the third-party software is optional. Nero 126.96.36.199d cannot be installed as a trial version. Nero 188.8.131.52a seems to be the last version of Nero 6 that had installers without bundled third-party (to Nero) software.
Nero 184.108.40.206, Nero 220.127.116.11a, Nero 18.104.22.168c, and Nero 22.214.171.124d all have been officially released by Ahead or Nero (while it is unclear whether Nero 126.96.36.199d was later removed from the Nero website and replaced by Nero 188.8.131.52c).
Calling Nero Customer Service on different consistently yields no useful result. One holds for a while in what appears to be a false queue while music plays, then the call is dropped at the Nero end before speaking to a Nero agent (after the queue has already reached one).
Nero 184.108.40.206d seems to be the most up-to-date version. Note This does not necessarily mean that Nero 220.127.116.11d was the most up-to-date version sold on discs in retail channels.