You are buying a re-manufactured CO2 capsule holder "charger key" to fit vintage and classic Merkuria Kovocas and Heimsyphon brand soda syphons.
This listing is for a re-manufactured CO2 capsule plastic charger/bulb holder and is designed to be used with the Merkuria Kovocas and German Heimsyphon brand soda syphons having a threaded part to the CO2 capsule socked with an outer diameter of 18mm (plus or minus 0.25mm). These syphons were produced in the period between about 1960 and 1990 although later re-badged "clones" were produced until much later.
See my other listings for period accessories.
P. S. If you happen to have a Sparklets soda syphon that you would like reconditioning, please see my other listings.