There is information about charges, but none regarding pressures here
“Report by Consul Robertson on the Manufacture of Fire-Arms in the District of Verviers and Liege – (1853) Rules of the Proof House, Gun-Proof Establishment at Liege, Belgium”, 1885
https://books.google.com/books?id=7EhJAAAAMAAJ&pg=PA132&lpg

Banc D'Epreuves Des Armes a Feu De Liege, 1906
https://books.google.com/books?id=5fxGAQAAMAAJ&pg=PA53&lpg

I haven't found shotgun pressures referenced until the 1924 revisions.