Short/middle/or long rail guidescope ring system. Includes two 120mm ID rings with three large stainless, nylon tipped adjustment thumbscrews, two dovetail bases that slide onto a rail that is 6 1/2" long. The rail mounts atop any dual rings that have a single top threaded hole. Attachment screws pass through slots on the rail. The slots provide an adjustable hole spacing from 3.5" to 5.75". So use this rail if your rings are between 3.5" and 5.75" apart. This is the plate for mounting the rail to the top of rings when they are mounted to the Stellarvue TP-6 or Losmandy DUPS plate.