This is were we still have some mystery. Bulova Watch Company didn't exist until 1923 I believe, maybe earlier.
I say this because we have an official Bulova advert from April 1923 that lists the company still as J. Bulova Company. We then see Bulova Watch Company adverts later in 1923. So following this the movement must come from 1923 at the earliest. My case theory for 1920 would still hold if we assumed a case manufacture stockpile in 1920/21. Just because the movement had the Bulova W. Co stamp doesn't mean it was manufactured in this year, just stamped and cased by Bulova in this year. Both case and movement could have been 'manufactured' in 1920/21 and then finally assembled and sold in one of the many many jewelers stores across the country in 1923.