Built for Union Castle Line as SOUTHAMPTON CASTLE. She and her sister were not fitted with derricks as they were designed solely for the Southapton-South Africa route and shore cranes were available in every port.
Derricks were fitted on holds 5 and 6 and cargo for St Helena and Ascension was stowed there.
The heavy lift derrick at hold 3 was seldom used.
When the ships were bought by Costa they were then fitted with cranes.

...fast mail ship would have slightly different cargo handling gear

Why so low on gearing? I would expect a classic 60s freighter to have topical forest of all sorts of cranes/derricks on board!