The psi are transducer numbers courtesy of Bro. Larry

This is helpful also

http://www.vintageguns.co.uk/articles/proof-sizes/
Chamber Length1925 Stamp..1954 Stamp..Service Pressure
2 1/2...1 1/8 oz. 3 Tons....8,943 psi
2 3/4...1 1/4 oz..3 1/4 Tons.9,682
3......1 1/2 oz..3 1/2 Tons.10,427
3 Magnum.........4 Tons....11,917