Dan,
The private viewers marks were not initially part of the Rules of Proof (I believe they are now). The Birmingham marks, indicating the year code and the grade of the viewer, started in 1922, stopped in 1942 on the retirement or death of the then Proofmaster, and resumed in July 1950.
In the meantime Proof continued exactly as before.
London did not start applying date stamps until 1972, and sensibly used the last 2 digits of the year ( apart from MM for the Millennium year 2000).
Birmingham went over to the last 2 digits of the year in 2006.
