I have to plead ignorance on that one. These valves have a conical plastic end that you twist to open/close. I had to do a speed test yesterday because in the process of getting in and out of the fly boxes I may have bumped this enough to cause a leak (I am assuming here; I got to shore and orally reinflated and it held air for the rest of the trip).
There is not any kind of cover that goes over the stem that I am aware of anyhow.