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 16mmSolid: 19 x 22.5mmSolid: 31 x 36mmOne Hole: 19 x 22mmTwo Hole: 31 x 36mmPack of 50.Please note: Extreme care must be used when inserting glass into the holes in rubber stoppers.