When I started in 1967 it was quite common for all to use registrations but it slowly became the norm to use flight numbers by 1969/70ish with perhaps the odd exception.
One day, doing Ground, a BYxxx callsign called for the airfield details, followed a while later by GAxxx calling for the same. A short while later BYxxx called for start, followed by GAxxx asking if they were clear to start. I then twigged and said 'Both the Captain and First Officer were cleared to start'. Much mirth all round.