This adapter is used for center poles that use threaded or un-threaded pipe, rods, wood poles, whatever.  It simply slips over the top of the pipe, and has a wood disk and a threaded bolt (3/8in) that sticks up through the tent's center grommet.  It has a eye-bolt to lock it in place on the pipe.  Very sturdy.  Fits up to 1 11/16" pipe or pole. Works with 1 5/8 fence rail just fine.