I gather that you are talking about the axle housing end flange gasket. Acutally, there should be two per side. One between the end of the axle tube and the back of the drum backing plate and one between the outboard face of the backing plate and the stamped steel bearing retainer (the stamped steel component with 4 bolt holes that holds the axle in housing).
You mentioned the gasket has a ridge. I'm guessing you are referencing a ridge around the large center hole in the gasket that fits over the axle bearing (?). Is the gasket made out of paper or is it made out of a very thin piece of steel?