This is the same model watch as the one Marty wears throughout BTTF II and at the beginning of Part III. It is a Casio CA-53 calculator watch. These watches are still being produced and sold today, more than 20 years after their appearance in the film.
The watch in the original film was a CA-50, which is very similar in appearance, but it's no longer in production and is much harder to find.
