If your barrel has a 4" red cap like the one in the picture then this is the cap! Yours may have two valves, the replacements now just have the one combined valve,. You can choose the pin valve for the throw away Co2 Bulbs or the old S30 type.
The pin valve allows you to use the small disposable co2 bulbs
There were actually some white caps that looked like this, and this will replace them as well.
Be careful as some red caps were 2" !
Now complete with a Stainless Valve