SCV For Selective control valve, I believe. Two for the Front end loader using the joy stick, and a 3rd added with often a lever on the right fender to control the rear couplings.
Cylinder I purchased from Carter & Carter Machinery in TN. Was an 18-26 cat I with check valve. I read that as it is an 8" throw with movement from 18" to 26".
Think that Surplus Center has similar cylinders as well, and others reading this may also chime in with other suppliers.
A search for hyd. top arms for 3ph on tractors should give a lot of hits, many here on TBN.