Outside of the three Piano 26s I mention below in a quote from another of my posts, there is the Cassotto, which is a separate model onto it own. It has an additional chamber on the back that makes it sound smoother and more like a flute.
Here is my quote:
“Hohner made three 26s: a Hohner 26 (yours) and two types of “Piano” 26s. The HM26 is plastic and looks more like a traditional rectangular-shaped melodica. It is one of my favorite melodicas to play, as hard as it is to play. But there is another Hohner Piano 26 shaped similar to yours. (See this link to view a photo of it: https://dl.dropboxusercontent.com/u/6237809/DSC_7981_edited-1.jpg) This Piano 26 is shaped similar to yours but is bigger and made of plastic. I love the tone and playability of this compact melodica, but it plays relatively softly. So for quiet introspective playing, it is one of my favorites. But it is not particularly well-suited for louder ensemble playing.”
So in reality, if you include cassottos, Hohner made 4 types of Piano 26s and 4 types of Piano 27s. The Piano 26s and 27s (HM models) are fast becoming my preferred melodicas because they bend notes well without damaging the reeds and you can get wah and other effects because the sound holes are on the back. They are hard to play well and you must hold them against you chest at an angle to boost the volume from them.
The Hohner 26s, 27s, and 32s (HM Models, plastic with nearly square bodies) are nearly extinct. Production stopped about 3 years ago. The Hohner Shop found a supply of them in their warehouse about a year ago and have been selling them on Ebay for a very reasonable price. No 27s are left, and only a few 26s and 32s are left.
I stocked up because these are the best “piano harmonicas” for playing the blues and sounding like a harp. I am going to publish a couple of recent clips on my SoundCloud page soon that show how close a melodica can sound to a harp. You won’t believe you are listening to a melodica.