Almost all reputable harp builders add larger cover plates (especially at the holes 1-3 end and for the plate that covers the draw reeds) on harps tuned to lower keys. There are exceptions, but good quality builders have typically done much testing to determine if this is necessary.
The reason is not to improve loudness but to give those long, low-tuned reeds sufficient space to vibrate without hitting the cover plates (particularly when played using lots of drawing power). An auxiliary benefit can be improved tonal quality and volume.
Yes sir. I hadn’t noticed that. I emailed Lee Oskar before I saw Slim’s response, after which reading it seemed obvious. This was his reply:
Hi Luke! Looking forward to catch up, let’s talk soon. Yes the bottom CP is deeper on lower key harmonicas due to longer reeds and therefore will not hit / slam against the CP. & also the sound is richer. Just like any instruments, a baritone/ cello/ tuba/ etc. lower octave instruments have a deeper chamber compared to a higher octave instruments. Compare a viola to a violin. Speak soon! Harmonically Yours, Lee Oskar
Thanks @p.alfarano for bringing this to my attention. You learn something new every day!