My 'hands on' experience dates from 1969/70, when I worked in the Prestwick duty room and then T3* check-in, then from 1974-80 when I worked in Western Routes (specifically USA) planning. I know the daily LHR-PIK-YYZ was all Y, I used it regularly to shuttle up and down to LON. (incidentally the PIK-LON passengers were segregated at the back, with a Customs officer to ensure they didn't interact with the transatlantics).
The year I spent on check-in, 1969-70, I can almost guarantee that was the only regular all-Y flight. Bear in mind that BOAC at the time had probably less than 25 daily departures from LHR. There wasn't even a Montreal terminator at that point - it went on to Chicago. Charters at that time were virtually unheard-of. The 747s should have been introduced during that period but they were sitting in Seattle with concrete blocks instead of engines.
I'd be a bit hacked to book the former and get the latter.
Probably true, but being told it was the royal aircraft might have helped.
* Sorry - 'Oceanic'. There were only two terminals at the time.