2. 1/4" x 8 tpi Backplate

Lathe Backplate.
 
  2. 1/4" x 8 thread per inch internal thread.
 160mm Dia with 131.24mm x 4mm Recess
 6. 5/16" Dia with 5. 5/32" x 5/32"  Recess, but will machine to suit.
 3 x M8 + 3 x 9mm Holes on 140mm PDC.