1/4" female thread x 1/4" compression elbow.
Can be used on New Autotrol Air Check for some 155 and all 255.
The use of 1/4" O.D. tubing to the salt tank is rarely used, but some older models were made with this size.
Most systems ( all newer systems ) use the larger 3/8" O.D. tubing ( Elbow PN 584564NO )