If you look at the " Member Resources " section on the Index page to this forum, you will find a thread labeled, " Articles, Charts and Links". I believe you will find a Chart showing common proof marks put on European made barrels there to use as a reference.
The Greater difficulty for most buyers of old shotguns is actually SEEing the proof marks. These are generally stamped with a metal DIE into the bottom of the barrels on Shotguns, covered by the forestock from view. The stamps are so small that you often need a magnifying glass to see them well. If the barrel has rust or pitting on the bottom as a result of its long age, you may have to do a Pencil rubbing of the mark to be able to distinguish the edges of the mark from surrounding pitting, or oxide.
( Pencil Rubbing: Put a piece of paper on top of of the surface with a mark, then using the side of the pencil's lead point to lightly rub over the paper on top of the mark. The lead "bumps" into the side walls and structures in the mark, making a more easily seen "picture" of the mark on the metal, but on the paper, which you can then hold up to the light to see every detail of the mark. This is also useful to "lift" badly corroded serial numbers, letters, inspection marks, regimental marks, etc. All these numbers, letters, and marks are useful in determining the origin of the barrels, their manufacturer, and age.)
i know of NO time in the history of the USA where Proofing of civilian made barrels was required by any law, federal or state, and NO PROOFING Standards have ever been adopted in the USA. You will find 18th century guns- muskets, shotguns, and a VERY FEW rifled guns--- made in the USA, but from barrels that are imported from Europe, which may have proof marks on the barrels. In the 19th century, it was very common to import shotguns made in Europe- Belgium had a booming business, no pun intended-- where the guns were labeled by the name of an American-Based company- often the retail seller of the guns, such as hardware stores. :hmm: