These radius blocks conform to the rotating ring system on the top and are flat on the bottom to mate with a flat equatorial mount plate. Felt lined. Came off of a heavy duty 10" F3.5 Parks Superior series newtonian.
Scopecity charges 122 shipped for what it identifies as a "radius block" (singular) but I assume is actually the pair (see under telescope making parts). My price is $60 the pair shipped in CONUS, paypal encouraged.