Hi Borja,
If you have questions, comments, or photos please post them and I will offer any insights that I can. I have already posted my review on the small-key Piano 26 and did note that this model comes in two variations: one with five back cover hold-dwon screws and one with three back cover hold-down screws.
I too am often surprised at how two melodicas of the same model can at times seem to sound and feel different if you really pay attention. I attribute it to variations in manufacturing and lots of materials as well as how a given melodica has been handled and stored.
Lowboy