I remember in about the mid seventies, one of the guys in our dive club showed up with a Horse Collar BC. Actually, it was a life jacket he stole from the Air Force but it worked as a Buoyancy Compensatory perfectly. Most of the guys in the club were some kind of Special Forces from the local SpecOps base and had access to all kinds of cool stuff. Whether it was issued or "liberated" meant little to them. I had to have one. By the time I got mine, many of the others already had one. All were OD Green except for mine. I stole mine from the Coast Guard so I got a cool bright orange one.
All of them has started life as a life jacket and came with a CO2 cartridge emergency inflator and a mouth inflator on the left chest. They worked perfectly as a BC and were nice to float with while waiting to get in the boat. Just blow it up and lay back and relax. Perfect except for one problem. Blowing it up was no big deal. Just remove the regulator from your mouth and blow into the tube on the left chest. Letting air out was a whole different story. You had to roll onto your right side, angle your body slightly towards the surface and then push down on the mouthpiece and hold it so air could escape. None of them were meant to be used that way but it was lots better than the old method of adjusting your weight from the bag of weights tied to the anchor.
Then one dive, I saw something different. It was a Horse Collar BC but it had a hose that ran from the top of the neck down the left side. It had some kind of valve looking thing on the end with a mouthpiece. What in the world is all that mess for? Then when we were all floating in the water next to the boat getting ready to descend and everybody is twisting up on their right sides to get the air out and he just lifted the hose in the air and presses a button!!!! Now that is convenient. A little while later, I saw him on the bottom adjusting his buoyancy. He just pressed another button to let air into it!! Magic! I have to get one of those, was my thought.
I ordered one from the Base Exchange and never looked back. That one even had a dump valve on the bottom so I could dump air from it while heading downwards. I still use one to this day. My only complaint about it, is the lack of gear stowage room.