As in the UK, every weapon that is marked with a proof-house stamp has actually been proofed in person, and examined post-act by the Proofmaster. He is legally obliged to comply with every aspect of the Proof LAWS in the country in which he is located. He is effectively guaranteeing that the weapon is safe to fire with regular loads, it having been tested to way beyond that safe load, subsequently examined and 'found fit to shoot'.
Weapons coming into the UK, from the USA for instance, have to be proofed before they can be sold to the public, and every weapon, new or old, that is sold to the public has to be 'in proof'.
Countries who do not need to have their weapons re-proofed in the UK, because of the historical maintenance of rigorous proof-house standards, include Germany, Austria, Finland and Swizerland. AFAIK, ALL other weapons intended for live-firing are required to be subjected to the UK proofhouse in Birmingham.
As the USA does not individually proof its products, all US-made weapons must be proofed before they can be sold in the EU, as well as UK.
UK black powder proofs can be seen on all my US-made BP weapons, and nitro-proof marks were on all my Section 1 handguns originating from the USA. Every chamber of every revolver is proofed individually, BTW, and failure in any one chamber scraps the gun.
Hope this helps you a mite.
Bestwishes
tac in Tokyo