It looks like the promised Onyx e-Music Score is less a real device than a placeholder for a similar device from one of Onyx's partners. At the very least there was no sign of the e-Music Score at the Hong Kong Electronics Fair last week, but several sightings of the Padmu.
This is a large-screen ereader from an Italian company, Outering. It is based on the older Onyx Boox Max Carta model (not the newer Max 2), comes with either one or two 13.3" E-ink screens, and costs $883 or $1754 USD.
That is considerably more expensive than the Max Carta
Max 2, which costs $550 on Amazon, but on the upside the Padmu ships with custom software designed for displaying sheet music. There's also a PadMu Controller app for Android which you can use to control the device remotely and open and close files.
I really don't see a reason to spend the extra $300 for the single screen Padmu rather than get the Max Carta; you can get nearly the same functionality out of the Max Carta by pairing it with one of the following BT foot pedals and using the Max carta to display PDFs:
And if you can afford the double screen unit then I think I just found my long lost cousin.