The screw-on plastic strip that covers the reed plates is different between the H and HP. I think the service manual calls it the “air box lid”, so I’ll stick with that (please correct me if I’m wrong). This is the piece that the mouthpiece plugs into at one end, and has the spit tube at the other.
On the H, the air box lid has a plastic lip that covers the area under the keyboard.
On the HP, the air box lid has no plastic lip, and the area under the keyboard is open.
From what I can tell, this plastic lip is a primary determinant of timbre. The two different metal back covers also influence timbre, but not as much as this lip, in my experience.
When I put a 44H air box lid on a 44HP, the 44HP sounds much more like the 44H (i.e. a smoother, less twangy sound).
When I put a 44HP air box lid on a 44H, the 44H sounds much more like the 44HP (i.e. a more raspy, more harmonica-like sound).
Here are photos of the air box lids I own. You can see the models/serials on the top stickers.
There’s another big difference between the H and HP, but I haven’t figured out what is responsible for it:
For me, it is _much_ easier to move air through a 44HP than through a 44H or a 44Hv2.
I feel as though I have to “push” air through the 44H, like I have to fight to get the notes to fire when and how I want. I find it very difficult to play fast fingerwork on 44H/44Hv2 for this reason.
The 44HP requires no “push”. Notes fire with the tiniest sliver of air, and I feel like I have much more control over how and when they fire.
The 44HP blows like the M37C or Pro37v3, while I have never played another melodica that blows like the 44H/44Hv2. It is extremely frustrating to play the ‘Gavotte en rondeau’ from Bach’s third violin partita on the 44H, while on the 44HP, the notes just “fall out” of the instrument. Even simple trills are more difficult for me to execute on the 44H.
My 44HP has…
– an air box lid from a 44H
– the solid metal back cover from a 44H
– a keyboard from a 44H
– one of the red ends from a 44H 🙂
…and while this gives the 44HP the smoother timbre of the 44H, it still moves air with the ease of a 44HP.
I’d appreciate any insight into what’s going on here, as I’d love to be able to modify my 44H and 44Hv2 to blow more like a 44HP.
One last note: air distribution in general is _significantly_ worse on my 44Hv2 than on my 44H, to the point where, for wide chords, I have trouble getting the high notes to fire _at all_ without overblowing the low notes. I don’t know if that’s a new ‘feature’ for the model, or a defect in my particular 44Hv2, but it is unpleasant (and unreliable) to play.
Hope this helps. Cheers!