Until 1956, the writing awards were no simple delineation between Adapted Screenplay and Original Screenplay. This was a time when studios marked a difference between their screenwriters and their story department. So: if someone came up with a story, its characters and plot and all that, but did not write the final screenplay, they were eligible for Best Motion Picture Story. If someone wrote a screenplay, with dialogue and action and INT./EXT. and all that, working off of either a play, a novel, a magazine article or a Motion Picture Story written by another, they were eligible for Best Screenplay. And if a screenwriter also wrote the original story they were working off of, they were eligible for Best Story & Screenplay.
Two-time Academy Award nominee Ruth Chatterton put it succinctly in her presentation of the Writing Awards at the 23rd Oscars:
