Here we have a complete listing of both hole end clock mainsprings as well as the loop end clock mainsprings. The hole end mainspring is the kind that is in a barrel and you cannot see the clock mainspring unless you open the barrel cap to see it. The loop end mainspring is the kind you can see right away when you look at the movement as it is not in an enclosure. It is called a loop end as it has a loop on the outer end that goes over the pillar of the movement. Both the hole end and loop end clock mainsprings have a hole end in the inner end of the spring where it will attach to the winding arbor.
There is a special tool required to change clock mainsprings, called a mainspring let down key set. This tool eliminates the possibility of the repairman getting injured and is described in the Clock Help pages. You really do not want to get whacked by one of the clock mainsprings, they can hurt. This is why the tension of the spring is released before separating the clock plates, or attempting to remove the spring in anyway. Without the tension of the spring being released, the spring will unwind it self very fast in a uncontrollable fashion that can harm the repair person and the rest of the clock movement.