These stoppers are used as an accessory for microscale threaded Standard Taper 14/10 kits.
- Pennyhead stopper sizes 10/18 through 19/22 are solid borosilicate glass
- Sizes 24/25 and larger are hollow borosilicate glass
- Manufactured from 33 expansion, low extractable borosilicate glass conforming to USP Type I and ASTM E438, Type I, Class A requirements