Australian delivered Minimatics (production ceased about 1969) did not
have an additional oil cooler, but they did have a different type of heater.

Instead of the heater temperature being controlled with a water valve,
they ran air through the heater core all the time for additional cooling
and controlled the temperature by varying the mix of cold air added.

Some much later UK Minis and/or Metros use an oil to water heat
exchanger to cool the oil. This fits between the oil filter and the
motor. Is this what the manual is referring to?

Certainly some form of oil cooler is a worthwhile addition to a
Minimatic in a warm climate, since that 5% to 10% of your power which
gets lost in the torque converter heats the oil.