Hurdle Maker's Twibill

This tool was used to make hurdles in jolly old England. Hurdles were, and still are common in the English countryside and are used in fences, gates, and other yard fixtures.
This Twibill has a sharp knifelike edge on one end to mortice the wooden hurdles, and the other end has a hooklike end to hook the wood and rip it from the mortice center.
The tool has a pin protruding through the blade end, and I have no idea why. This pin can slide back and forth, but cannot fall out, because it was peened on the ends. If anyone knows why the pin is there, please tell me. I never saw a hurdle Twibill with such a pin installed.