The new Kynoch website lists it as ".300 H&H Flanged NE", but doesn't include an illustration.
Roosevelt & Drake sell individual H&H-headstamped rounds for $6 a pop!

dnovo: You need to upgrade your shooting library.....!

Start with "History & Development of Small Arms Ammunition Vol.3: British Sporting" by George Hoyem. ...and Fleming's equivalent is a good addition for the more serious British sporting cartridge collector.

Hell, I bet even "Cartridges of the World" has the Super .30 Flanged!