"Developing or developed doctrine does not have to be written down. It becomes common practice by proven use. Another example is the French employment of open order or skirmishing tactics, of which there were various forms. These were not written down and are certainly not in the August 1791 Reglement."
Not to split hairs but its important to seperate 'Doctrine' and 'Common Practice' - especially when discussing earlier periods.
A technique may be part of common practice without being necessarily either correct, officialy sanctioned, or taught in any manner.
For a technique to make the transition from 'Common Practice' to 'Doctrine' requires official sanction and passing on by teaching. Obviously neither of these require writing specificaly but it sure does help in promulgation of the technique. To my mind this is where we make the transition from pre-Frederick 'common practices' to a in some ways post Frederick (although some aspects are much earlier) acceptance of sanctioned and disseminated doctrine required to be applied.
By the period under discussion I'd have thought we'd reached a point in time when to be considere doctrine a technique simply had for practical purposes to be written down - but of course that doesn't necessarily mean we have copies of it.