Not holsters, but how about first generation speed loaders from Safariland? Mechanical clips held them in place, until released by sticking your finger in the open end and pushing the button. I'm still faster with these than the newer ones. They don't rely on gravity to work, and can be used hanging upside down if needed. These have been in my "box-o-holsters" for about 40 years.
(http://i204.photobucket.com/albums/bb294/pioneer461/Guns%20-%20Pistols/SpeedLoaders004.jpg) (http://i204.photobucket.com/albums/bb294/pioneer461/Guns%20-%20Pistols/SpeedLoaders007.jpg)
(http://i204.photobucket.com/albums/bb294/pioneer461/Guns%20-%20Pistols/SpeedLoaders012.jpg) (http://i204.photobucket.com/albums/bb294/pioneer461/Guns%20-%20Pistols/SpeedLoaders011.jpg) (http://i204.photobucket.com/albums/bb294/pioneer461/Guns%20-%20Pistols/SpeedLoaders009.jpg)