Took a while to find this entry as it does not seem to be given in the IO Board GitHub readme. Maybe it could be added for clarity as I received my IO Board today and still no pins included?
So to copy N2ADR Jim's advice from another thread for benefit of others until they are included:
"For the three-pin header you need the same size as the 1x20 header on the main HL2 board. The pin pitch is 2.45 mm. On my HL2, the total height is 11.43 mm with 3.05 mm below the PCB. The pins are square and 0.64 mm on a side. But my HL2 is an old one, so check what size your pins are. The exact height is not too critical."
73
Max