All 26 GPIOs are buffered with bi-directional voltage translators to protect the RPi when experimenting with new circuits. Two different PCBs, a normal and a T-board version.
This version (PCB 150719-1) is designed to connect any PCB safely to a RPi. If connected by way of a ribbon cable K1 should be a normal 2x20 pin header. K1 can also be an extra tall stacking header so a HAT can still be placed on the RPi (3.5" display for instance). A ribbon cable can connect the output of the buffer to the external circuit. The use of 2x20 sockets (bottom side) is also possible.
Connecting the RPi side of the PCBs can be done by a short 40way ribbon cable with two 2x20 receptacle connectors if K1 is an ordinary 2x20 pin header. It is also possible to use a 2x20 female extra tall GPIO Stacking Header for K1 as most HATs do and place the PCB directly on top of the RPi.