I'd try a few drops of penetrating oil. Be careful not to contaminate the crystal gasket.
My 6306-7001 case doesn't have the collar you speak of. The top of the hole is peened over to prevent the ball from escaping. In order to replace the ball, you'd have to open the peened-over portion of the hole. Upon replacing the ball, I guess you'd have to use a closing punch to peen-over the hole once again.
Best of luck,