We spent an afternoon at his apartment in New York and I had a good chat with him about his writing for harp, including his trio for flute, viola and harp, "Sonata after Syrinx" (great piece) and he showed me his brilliant harp box tool which he always used when he was composing for keeping track of what key the harp was in as he was writing for it.
I've been making his little harp boxes and giving them to composers ever since as they are a brilliant aid for writing for harp - link to post here about how to make them -

I made a slight change to his design as with his, rather than have the pedals with flats at the top and sharps at the bottom he had his in reverse. When I pointed out to him that his diagram was the wrong way round to how the pedal box was on the harp, he replied, "my dear, I have them the logical way round, it's you harpists who think the wrong way round."