The simple-minded answer is that you should be able to play the melody for Amazing Grace on either harp. The only difference in the notes "contained" in the two keys is the F/F#, which "doesn't happen" in this melody - neither F nor F# is needed, and the rest of the notes are the same for either scale. The "span" of the notes in this tune is short enough for either harp.

The "real" answer may be that the tune is "modal," and the "key" is actually G-dipsolarian or something; but we'll have to wait for one of our people with some knowledge in that area for comment.

John