I found this question challenging, since I couldn't remember anything about "stock" or "stocks" in the story. I spent some time looking through the text and did not see the word "stock" anywhere. Finally, I copied the entire text into Word and then ran a word search. There is no "stock" in the text. However, the word "stockade" is used four times. Here is an example:
'I'm the Black Scout,' says Red Chief, 'and I have to ride to the stockade to warn the settlers that the Indians are coming. I 'm tired of playing Indian myself. I want to be the Black Scout.'
There is no real stockade. The kidnapped boy who calls himself Red Chief and Black Scout is only imagining things as he plays Indian. He forces Bill to pretend to be a horse while he gets on his back and rides him to the imaginary stockade which the wild boy says is ninety miles away.
A stockade is like a fort but not so elaborate. The pictures we see of stockades in books and movies all look pretty much alike. They are rectangular structures made of logs stuck into the ground. The floors are typically just bare earth. The tops of the logs are carved into sharp points to make it difficult for attackers to climb over them. There are usually gun holes in the log walls. But the stockade referred to by Red Chief/Black Scout is only imaginary.
I hope this answers the question.