I have an old book titled "The Hunter's Encyclopedia" which had a section on proofs. They stated, taken from German proof laws but the exact source not given, that min wall at end of chamber, regardless of length, for 12, 16 & 20 gs's should be 2.3mm (.0906") for ordinary good stel & 2.1mm (.0827") "IF" a special Steel was used. For the 24 & 28 ga's due to their higher pressures 2.4mm (.0945") was recommended as minimum. They recommended a min wall of .6mm (.0236") in the "Forward Third" of the bbl, with no other actual points listed.
I realize guns have "Passed Proof" with thiner bbls than these, but personally I feel more comfortable with any pre WWI gun if the wall @ end of chamber measures at least .100" (2.54mm).
Older guns having very light chamber sections I personally reserve "Strictly" for light BP loads.
