This adaptor screws onto the top front of your RRS or Wimberley lens foot to allow the B91-B flash bracket to be used with certain taller telephoto lenses.
Note about clearance: The FB-A600 raises the clamping surface for the flash arms by 6/10ths of an inch (.600"/15.2mm). There must be, at minimum, 5/8-inch of clearance from the surface of the plate to the barrel of the lens.