Several options are shown here for GL14 and GL18 glass threads.
Along with open and closed caps, there are septa for piercing and teflon faced sealing rings for various diameter insertions.
They will hold and seal tubing, probes, or electrodes with a seal ring with diameters about 6-7 mm ±1 mm from the nominal ID.
See the chart of GL thread sizes and seal rings for all thread sizes. The GL14 can hold inside diameters of 1.6, 3, 3.2, and 6 mm with a teflon insert.