Having been trained originally on the piano, I think this way. I visualize only one harp, a C harp in my case because that is what I carry with me and play with one hand while driving with the other.He thinks on a G harp and relates that key to the piano. He will recommend that you think on a C harp.
It's possible now that he tells people to think on a C harp but it's clear to me that its a G harp in the past.