For LED support with the Rev. 2 PCBs, the MOSFET and resistors for the MOSFET are available separately via the LED Support Add-on Kit.
Each half of the board is a 5x6 matrix, but there is the option to have a 2u key in the lower left and right corners in place of 2 1u keys. The spot for the 2u has holes for Cherry PCB mounted stabilizers.
There's also a spot on the PCB for a pushbutton switch to reset the Pro Micro for flashing. No more taking a wire/tweezers/screwdriver to jump the Reset and GND pins!
White with Black Text (Rev. 2)
Black with Black Text - Pro Edition - Gold Pads (Rev. 1)
The name Nyquist comes from the Nyquist sampling theorem, which says that if you want to sample a signal to convert it from analog to digital and be able to uniquely reconstruct the signal later, then the sample rate has to be twice the bandwidth of the signal. So here, if you want to sample this PCB, you're going to need 2 of them.
The Nyquist uses QMK for its firmware, and the code for it has been merged into the main codebase. You can find it here: QMK - Nyquist directory. The pins used on the Pro Micro are the same as the ones used by the Let's Split, with the exception of the additional row of course, so it is actually backwards-compatible with a Let's Split (if you flash both with the Nyquist firmware).