Large 5 1/4 in Diameter Dual Style Back Plate
 
This backplate was designed to accept either the General Foam/Empire style A19 light fixture with the flat metal connection plate, or the TPI/Grand Venture/Union A19 light fixture with the bent metal screw in connection.  
 
The backplate has the three screws holes for attachment to the blowmold.
 
This has my updated modification for added strength in the critical "slide in light fixture holder" area of the backplate.