Based on Lisa's theory, American Standard Watch Case Co hallmark (also without globe or shield) suggests that if this case may have been made before the trademarked date of first use (Aug 1918), however there is a chance that while Bulova used American Standard at that date, it may not have precluded Bulova from using other cases from ASWCC for a few more years? Perhaps right up to Bulova buying all of American Standard WCC (or date when it became a wholly owned subsidiary or what ever the verbiage was). We have 1920 date, which I'm guessing may be a little late for the various hallmarks. Maybe I'm splitting hairs (years) here, as I am not convinced its pre-August 1918 as per trademark info we have, but maybe it is pre Aug 1918?
Here's a 1919 Rubiayat w./ a globe, and movement swap, but its the case that Im comparing.