Learn English – the counterpart of an appendix

You are looking for the retronym ("a new name for something that differentiates the original from a more recent form or version").

The adoption of a retronym requires two things:

1. the new version being sufficiently popular
2. time

In the case of the smart-watch, neither has happened yet.

After reading through all the comments I'd like to propose addendum. Although mainly associated with publications it also serves in other area's.

something that is added; especially : a section of a book that is added to the main or original text