They were called FLUME TANKS,and they were built into the DART AMERICA and DART ATLANTIC,which at the time in 1970 were the largest British cellular container ships built at that date.That ties in with the approximate year(68) for the Port ships.
I don't think the system was too successful,but maybe Capt.John Arton,who sailed in them after sale to CP ,and a few years after me,could expand on this answer?
Gulliver