http://tos.trekcore.com/hd/albums/1x17hd/thesquireofgothoshd084.jpg Here we see the landing party in "Squire of Gothos" with some life support gear. This gear is mentioned nowhere on the Memory Alpha wiki, and nobody seems to have listed or identified the prop anywhere. I'm curious where else we might see it, and what was used as the prop.
Ah yes, I believe it is! So two appearances at least. And some fun details, too - I think I can make out "disposable" written on the side! xD That brings me a little closer in my search to finding out exactly what this prop is. Preferably, I would like to eventually find this exact kind of mask, but I imagine that might be pretty difficult, being that it's a "disposable" hospital mask from the 1960's.