I just saw a clip of that performance on youtube (such a resource!). Couldn’t find a credit for the harpist on the IMDB but if I had to guess, I would say the playing was by Robert Maxwell, because I have a recording of him playing “St. Louis Blues” that sounds just like that arrangement. Clifton Webb did a very nice job of faking. Maxwell (or whoever really played it) probably coached him.
David Ice is our resident expert on harp in the movies and probably knows the answer. I bow to his expertise.
I really like Robert Maxwell’s arrangements so I did a little more searching via google. There are some headlines and news clips saying Robert Maxwell spent two days prepping Clifton Webb. Rumor had it that Maxwell was paid $100 an hour. I wish I could get paid that now:) It was really a small fortune when the movie was filmed.