Vic,
All of the European ammunition and most shotgun ammunition manufactured by US companies here are CIP compliant and each box will have the mark of the proof house that tested the load.
Look for the "inspection of ammunition" proof mark on the box - they start on page 7 of this document
https://www.anarma.org/media/kunena/attachments/1233/genco_en.pdf . The most common I see on US ammunition boxes are Birmingham, Liege and Munich but I have seen others.
I would have no hesitation shooting US made 20 gauge cartridges with 7/8 ounce of shot at 1200 fps or less in a nitro proofed gun. My only concern shooting 1 ounce CIP compliant loads would be recoil damaging the stock not a pressure issue; recall that recoil is mass of ejecta and velocity related and has nothing to do with pressure.
