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The picture in Vall’s post is not visible on this side of the pond, but I assume it was similar to this box.
This is Kynoch made ammunition loaded with streamline bullets for Match rifle competition at 900 to 1,000 yards.
Brought in during the 1930’s, discontinued during WW2 it was reintroduced post-war until .303 was supplanted for Match and Target Rifle competition during the late 1960’s and early 1970’s.
The blackened case heads were apparently to indicate that it should only be single loaded as being insufficiently crimped for use in the magazine.
The streamline bullet led to the development of the military.303 Mk. 8z round for long range machine gun fire.
As far as the War Office was concerned “.303” ammunition was just “.303”.
“.303 British” was applied to commercial ammunition to avoid confusion with the .303 Savage round and a few relatively scarce proprietary cartridges.