In March 1964 it was agreed that the five accommodation decks would be named Main, Upper, A, B, C. This was in keeping with the original Queens.
In April 1964 this was amended: Upper became Foyer.
By December 1966 the final naming scheme (One, Two, Three) had been agreed.
So the original names and final names were:
Observation Signal
Sports Sports
Boat Boat
Verandah Upper
Promenade Quarter
Main One
Foyer Two
A Three
B Four
C Five
D Six
E Seven
F Eight
In answer ri the original query the practice of 'Deck 2', 'Deck 4' as opposed to Two Deck and Four Deck ony started with QM2. and then the numbers went upwards in the ship rather than downwards as on QE2.