If you are interested Glider90 it was a similar setup on Victor tankers. Checking my logbook I see numerous entries where I flew single nav (like P-N I was a nav radar too). These were usually trips like pilot IRTs, air tests etc where there would be no refuelling so one navigator was sufficient. These trips were usually given to nav radars so that they could keep their hand in doing some actual navigating/plotting, as opposed to operating the radar and the refuelling kit. Later on of course the Victor K2s were converted to single nav and operated with a four man crew, which made sense.