We could compare `build_index` to `(self.build_number - 1)` which is how it is currently made up (see https://github.com/Nebelhom/mozdownload/blob/871ccc94d8aeed439be827ca0377162cf8f521c4/mozdownload/scraper.py#l342 for details).

Not sure how meaningful this would be. Maybe if someone is trying to change the code at a later date it could be useful.

What are your thoughts?