26 pins out of a Pro Micro
A Pro Micro, three pin headers including 2 pins for an optional reset button, and finally 8 loose forked pins. These can all be soldered together to prepare the MCU for use in a Concertina v0.7.0.
See Concertina v0.7.0 test and Concertina v0.7.0 assembly, closing the case, pictures of the MCU with these pin headers in place and in use. The forked pins, soldered onto straight pins, enable GPIO ports A3 through 10 to receive key columns from both halves of the Concertina via M20s.