In contrast to other development boards like the Pro Micro, Elite-C, and dozens of other controllers featured on this blog, Stampy gets soldered directly to 3 switches, meaning that no extra space below the controller is needed.
Another sign that this devboard was designed specifically for keyboards is that 3 pins are preset for columns, and 1 pin is preset for a row, so in fact you get a 3-button macropad PCB.
16Mbit (2MB) of flash memory
26 available I/O pins
USB Type-C connector
GPIO pins available: 0-8, 10, 11, 14-28.
4 of the pins are connected to the 3 switch footprints: GPIO14 (row0), GPIO11 (col0, left switch), GPIO10 (col1, center switch), GPIO15 (col2, right switch).
The reset button is connected to RST and 3.3V (not GND).
Due to the tight spacing of components, you will have to clip some plastic pins of 5-pin switches.