This is a spacer that allows you to use a 38 mm bottle cap on a bottle where the bottle thread is too long to allow the cap to get tight. This comes with an EP O-Ring installed which makes it seal to the top of the bottle. 4 liter bottles from VWR have a locking ramp made into the glass on the neck of the bottle just below the threads. This locking ramp can interfere with the bottle cap from an instrument screwing down far enough to get tight and seal. This spacer allows the bottle cap on your instrument to get tight and make a seal onto this newer type bottle with the locking ramp.