They used to use these at Parris Island, but I remember seeing these all the time when I was at Kadena in 69.
I worked comms at the air terminal and we’d get contracted commercial airflights of Army and Marine Corps troops in and they’d be transferred to these to go to jungle training on the island - then to Viet Nam.
Also would see the troops filtering back from Viet Nam brought in on C-130s, etc. and then put in them again to be taken to get cleaned up and sent back home. I’m a Georgia boy, and I never thought I’d see that red Georgia clay outside the state, but I saw plenty of it from those troops coming back from there.
It was caked on many them; dirty sweaty, tired, their boots worn and flattened - yet there was a broad smile on all their faces just glad to get back to the world (or on the way to it).