Some of Angle Oar's mounts incorporate RAM Mounts products, including this RAM Spline Post Adapter.  If you're using the Twin Track Mount, for instance, you may want to purchase a second support post in another length, in which case you'll also need the spline adapter.  
 
This spline adapter consists of a spline post with female PVC slip pipe socket. The socket can connect to any any of our support posts or RAM's PVC pipes. On the other end, it slips into one of many different RAM Mounts base mounts, including those featured in the images above. 
 
Purchase includes spline adapter only. Support posts and base mounts sold separately.