I bought my belt used. I am unsure of the tube size on the "front" it is 4 inches. On the back it is 3 inches. I am guessing it is called the 4 inch tube
The longest end calls. But to make it for sure, put the whole belt together separate from the body. When the tip (end) of the cage almost reaches the outer cage you have the 4" tube. When there is 1" (or 2,54 cm) space between the tip of the cage and the outer cage you have the 3".....