I've been trying to get a handle on the twang word. Looking around on Google books I've discovered the word THWANG.
Of Saxon origin, the word thwang seems to have two uses;
A) a large chunk of bread or cheese.
B) it was a leather thong used to secured boots to your feet.
So Thompson's mother may not have been to far from the truth when she described the twangman as a dealer of twine.
I could speculate that a thwangman made or sold strips of leather for tying shoes to the feet (no eyelets and laces in those days). That scenario might make sense of the terms twang-cart and twang-knife.

Fergus Russell