3/8" female thread x 3/8" compression elbow.
Used on some threaded "pick up" assemblies. And used for the Autotrol Performa brine connection.
br>Note: Do not confuse this with the commonly used 1/4 f x 3/8 c elbow used with the newer Autotrol Air Check Valves, and older Autotrol 168 brine connections.