S51 CSV APO Product Information The S51 CSV APO had an Apollo (APO) lever that was slightly smaller than the commercial grade S-series Saturn (SAT) lever. The inside mounting screws on the S51 CSV APO were exposed like the F-series. Originally sold to Home Depot and Lowes as a contractor series. This product was discontinued around 2011. The cylinder part number used on the S51 CSV APO was 21-033. This was a zinc cast 6 pin cylinder keyed to a 5-pin key Replace with F51 ELA The Commercial S51 has concealed screws Cylinder is brass not zinc – not shown here