The Apotar lens on mine has an aluminum outer ring on the front of the lens with the distance markings on it. This ring is attached by three tiny set screws around the edge that screw in radially toward the lens axis. These screws grip a barrel inside the aluminum ring. You might just be twisting the aluminum ring without turning the barrel inside, which is what actually contains the lens elements. If you have a set of jewelers' screwdrivers, you can loosen the set screws until the aluminum ring slides off, and then you will have access to the inner barrel. Then you can try penetrating oil or use a hose clamp as a wrench and try to loosen the inner barrel.