There may be more than one correct style, as Jeff's pics show.
I have a few in known original pans and one has the mark "F - C" and the other is mostly flat but has the circle raised embossment area where the bolt shank would be. They both have a "pilot" on the threaded side.
The original nylon washers appears to be fairly thin -- thinner than those you buy at the box stores (left in pic 2).