This 20mm sleeve stopper is manufactured from natural red rubber and is made for use with vials having a mouth inner diameter x outer diameter of 13 x 20mm. A sleeve stopper is a plug stopper with a fold-over skirt that is used on serum vials and other crimp-finish containers that do not require an aluminum seal or overcap, and is often used as an injection site for multiple dose entry. The general-purpose formulation of the natural red rubber has excellent resealable properties, making this sleeve stopper ideal for use in multi-puncture applications. Stopper Style: Sleeve. Stopper Material: Natural Rubber. Color: Red. Sleeve Stopper Size: 20mm. Vial Mouth ID x OD: 13 x 20mm. Wheaton #: 224100-161.