I’m not entirely unconfused myself, Daren! I don’t think the reed cover has anything to do with it, although it’s a bit different. My impression is that it is the external shell that’s making the biggest difference in sound, due to its thickness and perhaps due to a difference in the plastic. The end pieces are also flatter on the P-37E. My guess is that the sound is muted for the entire keyboard but that the muting is more problematic at the high end. More air pressure is, it seems, required at the high end of the P-37D and other melodicas. The muting by the P-37E case exacerbates this issue. To confirm this, I should take the core out of my modified P-37E and see how the high notes play.
But I already have been familiar with how the P-37D plays outside the shell, and I’ve already taken the damn thing apart so many times, I hate to do it again. But eventually I’m sure I’ll take it apart to see if there’s anything I can do to improve the situation with the high reeds.
I really like the sound of my altered P-37E. It’s special. But I’m likely to avoid the top end keys.