1. 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 
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