Missing Something - I have no excuse. I have Chris’ book and I just didn’t go look at it. I was going to answer the question the same way you did, and then thought better of it, since I knew that yellow seals, etc. meant some other things in other countries - Ballistic Standard in England, Proof load in South Africa, Armor Piecercing in Dominica. I used to have the 8mm Mauser version, which was observation, and I may have had a .303 - don’t remember now, as been awhile since I sold that collection.
Admittedly, even though in this case I couldn’t take time to look it up, generally buying a cartridge lets you learn about one cartridge while buying a book on them lets you learn about hundreds. That’s why I have a big library on arms and ammunition. Can’t keep in all in my head - heck, can’t keep much of anything in my head anymore.