Large Hollow Laboratory Glass Stopper

This large hollow laboratory glass stopper (also known as a ground glass joint) is commonly used in laboratories because of the nonreactivity of its borosilicate glass. This unusually shaped 3.5" high stopper has a 1.875" in diameter vertical round top (0.5" thick) connected by a 0.5" stem to a 3" in diameter (at center) spherical base with a flat bottom.  The bottom of the base is frosted and rounds down to 1.25" in diameter.  The top is marked with a butterfly-shaped outline symbol and an interior number 50.  Manufacturer and age are unknown.  This large hollow laboratory glass stopper is in very good condition.