Another name that we were considering was `original_indices` or `original_ids`. By the way this name should probably be plural, so if we stick with epochs better to do `epoch_ids`. If we want to go all out we can do `original_epoch_ids`.

While we're brainstorming I was going to suggest `original_event_ids` as well, but then I realized that we use event ID to mean something different, so maybe ID isn't perfect anyway; maybe `original_event_indices` / `original_epoch_indices` / `original_epoch_number` / `original_event_number`. (Just putting a million options out there to see which one appeals to people who don't already know exactly what we're trying to achieve)