The RAM Mount U-Bolt adapter changes the RAM-B-231Z and RAM-B-231 U-Bolt Rail Mounts to a 1/2'' (12.7mm) diameter.
The U-Bolt that ships with these Rail Mounts is design to clamp to a round rail with a diameter from 3/4'' (19.05mm) to 1'' (12.7mm). The RAP-319 rail adapter narrows the rail to a minimum with of approximately 1/2'' (12.7mm).