Currently in pre-order for $349 and begins shipping March 2024.
As far as I can tell, the only differences from the original are the included 2.5mm MIDI Out and DNA connectors are only on the sides (no longer on the back as well).
[EDIT] from the FAQ:
What is the difference between Seaboard BLOCK M and the original Seaboard BLOCK?
Seaboard BLOCK M differs from the original Seaboard in that you can now control other hardware instruments with a new MIDI out. There is also advanced customization of keywave response through updated firmware, offering more nuanced MPE control.
Seaboard BLOCK M has a more natural, intuitive playing experience with an improved velocity response curve, and durable design that has been refined to withstand the rigors of touring and extended studio sessions.
Go computer-free with mode-switching on your BLOCK M, so you can change between Single Channel, or MPE and other modes once activated in Dashboard.
Switch between your saved Equator2 or ROLI Studio presets by holding the side button and pressing the octave left and right buttons. Select your own ROLI Soundpacks to get you started with MPE.