This gasket fits into the center probe recess of a American Sankey Coupling Head (our item D44). These should be installed so the stepped side is facing outwards. In other words, install so you can see the step on the inside of the gasket, not with the step side down so you cannot see the step when the gasket is installed.
These are a bit tough to install. Be sure to use Keg Lube or another food grade rubber safe lubricant on this gasket when installing. We find it is easier if you first heat the gasket for 30 seconds in 150° F. water (to soften it), then lubricate it with a bit of Keg Lube, and then finally press it into the center circular coupler groove. Press in one side with your fingers, and use a small screwdriver to get the other side in.