I changed out my derby cover, and found a ring not round undercut into the original cover. Useless to the new cover, you need the paper cover from HD.
Just a heads up.

 

I don't know this for a fact but have heard rumor a few years back...

HD is trying to stop the production/need for the thin red gasket on a derby cover. Soon all derby's will need the disc gasket.

This is third hand info.

Experience says it's about 50/50 when I pull a cover. Could be either. Aftermarket covers are usually disc gasket.