This #7.5 Drilled Rubber Stopper (DRS) has a top diameter of 1 1/2", a bottom diameter of 1 1/4", and has a 3/8" hole drilled in the center, the perfect size for an airlock.
The #7.5 rubber stopper fits most 1000 ml Erlenmeyer Flasks and is also available as a #7.5 solid rubber stopper.
