You need to place your head and eyes against the tub, rotate the stopper and you'll find a screw. Loosen it to remove the stopper. It might be a conventional screw or one requiring an alan wrench.

I have one like that about that vintage and it has no screw. If you keep it the up position and unwind it (the opposite to the way you turn it close) it will eventually unscrew. It's a bear to get it threaded back in properly, though.

So just saying that if you can't find the screw, there may be a reason for it.