Hi Alan Et Al.,
To the best of my knowledge the HM32 and Piano 32 are one in the same, but I could be wrong on that. Here is my observation: all of my Hohner “piano” 26s, 27s, and 32s (and I have a lot of them) have various logo and water valve designs. Beyond that, all sound the same and use the same materials and design on the inside. I have taken about 5 of them apart for cleaning and repair. There are three different logos and two different water valves shared among my collection.
The cassottos obviously are of a different design.
I have some of the latest stock in my collection and I have some pretty old ones too (based on the case design, case color, and case condition).
If someone knows of a difference in design in this model line, I would really be interested in learning about it to add to my knowledge.