Difficult !
“Block E” seems to have been approx 1960-61, when PYY was leased in from Cubana
The scheme appeared also on some Vikings, and at least one Viscount VR-BBH
Interesting that G-ARKA, the next Britannia into Eagle had the same nose area cheat line as PYY, but the later curved E on the tail
From pic in post 6 I would suggest that she flew with “Block E” and your card was made up by an advertising type person (ooh dear I’ll never be PC) who added Cunard and modded the tail