Got this Hensoldt Wetzler scope, no model number I can find. It has elevation adjustment on scope, but windage is adjusted by moving rear ring mount. Question is this: It has a + - ring for magnification, but I'm not sure how to move this. There is a screw on the ring, and, I loosened it, but still can't turn the ring. The screw does not come out. Anybody have one of these, or any information on same?