Can anyone tell me--definitively--when the first drama written specifically for radio was?

I've looked at three different sources ("Writing For Radio" by Rosemary Horstmann, "The New Shell Book of Firsts" and wikipedia.) and they all say different things. Ones says 1922 one says 1923 and one says 1924. Anyone have a real idea?
Oh and I know about all the stage play adaptions and the sketch in 1991 but I want the first written for radio drama.