Kombi:
It is sweated on with solder. Get the sight leaf out, push down at the hinge area and pull back toward the receiver, it is kind of tough to do. Under the leaf there is usaully a screw, remove it. Now heat the sight base till you see the solder start to flow and then drive it forward with a hardwood block. You will have to remove all the excess solder from the barrel before you can reblue it, just polish it off with some emery cloth.
Rebel49