IIRC, in the early 60s the copilots had to have had a previous tour typically on Canberras. In 1964 we started to get abo copilots but they were with typically 3rd tour captains.
About mid-tour and abt 450 hrs o type the co would do an intermediate copilots course and then picked up for a captaincy at the end of the tour.
Add in an OCU and he would have had to be a st mover to have much under 1000hrs. Once on the sqn conversion would leave him pretty close but it would have been sensible to consolidate his own training. I know copilots were separately checd out by sqn QFI/IRE with at least one fatal and one near miss so a 1000hr rule would have seemed sensible.