I found this description in the brochure regarding Steinway-made Boston pianos. I am asking for clarifications and details on this system which, according to them, greatly enhances and enriches the harmonics.
But even other pianos, Steinway itself, possess the duplex scale. It is not a novelty. It serves to further highlight the harmonics of high notes. The matter would be more complex. But let's settle for knowing that it is useful, making the harmonics of high notes more audible, which would otherwise be lost and make the sounds themselves appear "flatter"