I believe that was the fake Mid-Atlantic or Transatlantic accent the “beautiful” people at the time created to set themselves above the “little” people.
You’re correct. Another actress who talked that way was Grace Kelly. She and Katherine Hepburn in my opinion were the most overrated actresses in Hollywood. Kelly always set my teeth on edge with her fake plummy accent. She was an Irish Catholic from Philadelphia, and her father was a building contractor or brick layer if I’m remembering correctly. Nothing wrong with that, but don’t affect an accent that suggests you’re a member of the upper classes or British royalty. Then, to cement her “otherness” Grace Kelly married a prince and left Hollywood. Good riddance.