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, such as the Canon 400 mm/f2.8-LS lens.
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.
Length= 1.5" / 38mm
Width= 0.7" / 17mm
Height= 0.6" / 15mm allows use of B-91 with Canon 400 mm/f2.8-LS lens.