The article below should be helpful in understanding how the guards are constructed:
http://iainnorman.com/articles-2/23
The rivets hold the parts of the guard together, and sometimes also serve to attach decorative brass plates.
The grip is a hollow cylindrical tube, typically made of sheet metal. The pommels are also hollow, and the tang is peened to them, holding the whole thing together. In some instances the entire hilt was cast, but those examples tend to be associated with ethnic groups on the periphery of the area of takouba distribution. In some of the older examples the 3 separate hilt elements are forged from steel.