What is the Derby cover on a Harley Davidson?

Harley Davidson "Derby Cover"

A "Derby Cover" is basically a dust cover. It's on the left side of your bike and covers the business end of your clutch. It's the larger cover with (usually) 4-6 screws holding it in place. However my '85 Sportster has no screws in the "Derby Cover". Instead there's a two inch (2") screw-cap in the center of my "Derby Cover" with a large closed slot in the center. That's it! there are no other screws holding it in place.