At one time this very special glass had a thin rubber like dental dam glued to the bottom and up around the top edge of the glass by about 1/4".  This was painted black on the visible surface of the glass and the black paint went up the stem just to where the glass body begins.  There is a very small opening near the lip of the glass and the glass is double wall.  In this manner, liquid could be shown in the glass at first  and actually drank.  After drinking the glass could be covered for a moment with a handkerchief (not supplied) and when removed the glass would be shown full again!  This was due to liquid being contained in the bottom section of the glass and by pushing on the rubber on the bottom would cause the liquid to come between the double wall part of the glass.  For the Collector of Glass Magic this is a very fine piece.  If desired, one could take the time and make it usable if desired.  Condition First Rate.