WATCO tub drain
 

I am (unsuccessfully) trying to remove the push-pull stopper on a WATCO push-pull tub drain. I can't seem to figure out how to remove the stopper. I am sure that it is somewhat corroded. It looks like there may be a hex under the stopper. Size unknown (less than 3/4").
Help.
:confused:

The knob on the stopper should unscrew. Hold the stopper and just turn the knob on top. Underneath the knob is another screw.