This flange is used with fiberglass holding tanks.  It is fiberglassed on to the tank.  A Thetford waste valve can then be bolted to the flange.  

The main photo shows the flat top that goes against the tank.  The second photo shows the bottom of the flange which is a standard 4-bolt Thetford mounting surface.  Any of Thetford's flat-top valves will work against this mounting flange.

Thetford PN 01662