Donald has wanted for a long time to have a tremolo harp that would allow him to play a mixolydian scale (major but with the 7th note in the scale lowered) in first position, to match the highland bagpipe scale. It would be like taking a diatonic harp and lowering Draw 3 and Draw 7 by one semitone.
You could get the same scale by playing a D-harp in second position. So why bother to retune an A harp?
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