AIP ENR 1.7-5 (my bolding):
3. CRUISING LEVELS
3.1 Selection of Levels
3.1.1 Flights must be planned in accordance with levels selected from the tables at Section 5. Any part of a flight that will take place south of 80°S must be planned in accordance with levels selected from the tables at Section 6.
3.1.2 Within controlled airspace, ATC may assign and pilots may request a level that does not accord with the tables in Section 5.
3.1.2.1 Pilots must only request a level not conforming to the table of cruising levels when it is determined by the pilot in command to be essential to the safety of the flight and its occupants. In such circumstances, the phrase "DUE OPERATIONAL REQUIREMENT" must be included with the level change request.
3.1.3 ATC will only assign cruising level s not conforming to these tables when traffic or other operational circumstances require.
I don’t think it’s about the absence of briefing offices,
LB I think it was a factor, contributing to
the slow mediocritization of the instructing system
Pilots - in particular new instructors - were largely left to themselves to interpret requirements. They passed their understanding onto students, some of whom then became instructors themselves, who promulgated the same interpretations on to their students etc. etc.
Something like the old "
Send reinforcements, we're going to advance" >> "Send three and four pence, we're going to a dance".