There are several posts about Williams Fire Sights on Rem870.com:
Williams Gun Sight Company: Remington 870 Universal Ghost Ring Firesights
Williams Gun Sight Company: Remington 870 Fiber Optic ACE IN THE HOLE Sight Review
Remington Ghost Ring Firesight Set by Williams Gun Sight Company, Inc.

I decided to try another type of Williams Fire Sights on my Remington 870 with rifle sights: