Casio CA53W-1 Calculator - Finn Wolfhard - Stranger Things
In the first seasons he wears a calculator watch as well, but not the classic Casio. But based on one of the first promotional images of Season 3, it looks like he finally wears a real Casio CA53W-1 Calculator. We can just make out the shape of the case, the Casio logo and the dark colored buttons.
The Casio CA53W-1 Calculator watch was made famous in the 1980s by Marty McFly in the Back to the Future movie.