I'm not so sure about "knee-irons" (or "nicky-tams") being used to keep the bottom of the trousers out of the mud (after all, one could simply roll up the leg), I seem to recall seeing the same technique being used in old photographs of harvesting. I was told, when I was a lot younger, that it was used to stop startled vermin (rats etc) running up the trouer leg (of couse, the uncle who told me could have been winding me up).

Good Luck

Tom