That isn't a bad site to read. Probably the best coverage for British proof of shotguns is in the back of Brown's "British Gunmakers, Volume II". It's important to understand each mark, but that barely gets you started. When there is an issue, you won't catch the easy ones. What's most important to understand is how the collection of marks is used as a system, and what changed each time new rules of proof were promulgated. I've never really found a reference work that does that well. It takes practice. Make note of the proof marks on every English gun you pick up, and see if you understand them without the book, then check. I've never found a dealer that was really expert on them.

Once you get the hang of it, you'll understand that sometimes marks that are missing tell you what you need to know better than the marks that are there. With this Hussey, the question was re-proof of original barrels, or new proof of new barrels. The Re-proof Mark was absent, which rules out re-proof of original barrels.

British proof is actually pretty simple, but if you need a book, you need help, and it's important to understand the marks on the gun before you buy.

"Serious rifles have two barrels, everything else just burns gunpowder."