this is just a wild guess mind you, but maybe for holding a young tree straight when planting it so it doesnt grow crooked? .....thats the best i could come up with
Definitely a traction splint, used on patients with a fractured femur to prevent the bone from obstructing or damaging the femoral artery, as the tension of the muscles tend to pull everything "upward" when there is no intact bone to hold the leg's form. I've used these several times in the field.
Here's a pic of a modern one to give you a better idea of what they look like when used: