Ah "rattler" that's so cute, I'm guessing you're referring to the widget that releases the CO2 to give it that creamy head.
It's the one in the purple can with the little rattler thing in it. Young's is probably what I meant.
I'm guessing g Brett meant Young's Double Chocolate because Murphy does not make a chocolate stout.
I'm pretty sure it's nitrogen and beer, not CO2.