Nitrogen regulator and want to use it on a CO2 air tank this is the unit you will need.
Draft Beer Nitrogen to Co2 Adapter.
This will work great for your Guinness
Brass construction
If you already have a Nitrogen regulator and want to use it with a CO2 tank, this is the piece needed to make the conversion. * Specs - Dimensions: 2"L and Connects CGA 320 valve to nitrogen regulator.