Following WW-II BOAC recruited pilots of similar age and experience, all jolly good chaps who had been recruited into the Air Force in more or less the same short time span, so who should be Captains and who co-pilots ?
It was even before my time, but I believe it was more or less first through the door were Captains, and then the rest became co-pilots, so that this latter group would reach retirement - then age 55 - at the same time as those lucky enough to get the command slots, and never get promoted, so finally, after a few years a deal was struck whereby for every really old - i.e. 55 - Captain 2 co-pilots were promoted. They were called Captains X and would often be rostered together, one in command outbound, and the other in command homebound.
I was a P.3 / Nav on the 707 whilst this procedure was still working its' way through the seniority lists, and one day flew with a left hand seat Captain X who announced that this was his first trip on his own after just completing his conversion to the 707, and asked the crew to ' keep an eye on him ' Early CRM !
With no serious navigation to be done between London and Rome, I read the newspapers and then climbed into the P.3 jump seat for the approach to Rome, to notice that nothing had been done by way of approach checks, and that the right hand seat Capt X was silently tapping the check-list on his knee. I prompted the L.H. seat Capt X who then called for the approach check list.
After landing he asked me if I needed to navigate between Rome and Singapore, and on the route we would follow there was no pure nav. requirement, and he then asked the R.H. seat capt X why he hadn't reminded him to start the checks - would he have allowed him to land gear up for instance ? Of course, was the reply, you're the Captain, you didn't ask for any checks. !! At which the L.H. seat capt X told him to f**k off to the cabin for the rest of the trip, and I would be co -pilot from there to Singapore.
Of course I wasn't exactly relishing the trip home, when the Command roles would be reversed, but whilst flight planning at Singapore the original R.H. seat Capt X was berating the staff as to why they hadn't briefed him before we left the hotel, as was the habit. The original L.H. seat Capt X said " I was de-X'd last week, I'm still in command, f**k off again.
I was co-pilot all the way back home, too !
Happy Days, but the above scenario, even with the Capts. X alleviating the problem a little, meant that many co-pilots were only promoted shortly before retiring, I waited 16 years, to over age 40, and was actually luckier than a lot.