Best quality red rubber.  Shore hardness in range 46-55.  Bottom diameter moulded on top face.  Resistant to dilute acids, alkalis and ammonia.
Includes packs of 10 stoppers of each of the following sizes:

Solid: 13 x 16mm
Solid: 19 x 22.5mm
Solid: 31 x 36mm
One Hole: 19 x 22mm
Two Hole: 31 x 36mm

Pack of 50.
Please note: Extreme care must be used when inserting glass into the holes in rubber stoppers.