This brass bull-nosed adapter is commonly referred to as a 'Co2 to Argon bottle adapter'. It allows you to use a Co2 gas regulator with mixed gas bottles (standard for beer dispensing in Ireland) such as 75/25 or 50/50.
The male 5/8 BSP thread on one side fits industry standard mixed cylinders - such as that of Star Gas, while the other side has a male thread to fit the female connection/nut of a CO2 regulator.
This is a great enabling piece if you are looking to move from/between 100% carbon dioxide cylinders and cylinders with a nitrogen split.