This is a clamp with inside diameter of 59mm. It has 20-1/4 threaded holes at the bottom. Its base is a standard 38mm Arca Swiss tripod plate. You can use it with standard manual Nikon extension tube commonly found on the market (but please ask seller for dimensions). It is 11.72mm wide, fits the 28mm length tube.
 
The benefit of using Nikon extension tubes is that it has large enough inner diameter and grooved inside to prevent internal reflection.