“Use by” dating of .303 for Aircraft ammo was common up to the end of WW II, and even later (Canadian Mk7z in the 1950s)
Aircraft ammo had top have reliable Primers, hence the extra high quality primers used in “Air Quality” ( more commonly known as “Red Label” ( the typical British Military label was printed in Red, rather than Green or Black).
It was known that primers deteriorate in time, so usually a
"one year" Use by date was printed on the outher crate or can.
The other rule was that any ammo used in a plane flying above a certain height, and returning to ground, that ammo was relegated to “Ground use only” as the Cold and Low atmospheric pressures affected the reliability of the priming compound.
Reliability of priming compound was especially important in planes with “synchronised” guns ( to fire throught the arc of the propellor)…in fact the Limitation on this ammo was [color=#FF0000]“not to be fired in synchronised Guns after dd.mm.yr.”[/color]
Finally, in British ( and Empire) service, ammunition for “Air Service” was headstamped with the Entire Year ( four digit) date; Land Service only was with a two digit date. By 1943-44, the Quality of priming compund had increased so much, that All .303 ammo ( Land and Air) was simply dated with a two digit date, BUT some Lots of 1945 ammo ( R^L) for Greece had the four digit date ( and Cans were “Red Use-by date” stamped accordingly)
In the Case of US contract ammo, whilst Remington had the stamp " .300Z" on its .30/06 ANM2 ammo for Britain, the WRA .303 did not have any “Z” in the headstamp…simply a year dated ( Four Digit) WRA stamp. The WRA ( as shown recently) packets were Red Print on white carton for Aircraft Quality, and Black print for Land Use.
As to the “not functioning in Brens” that sounds like a furphy ( unreliable info)… .303 Ammo for all services was loaded to the same QA limits…only that for Air Service, the quality of the primers ( sensitivity, reliability of ignition under severe conditions) was the only “difference”…Air Quality ammo used both Cordite and Tubular grain Powder in British Loading, and Tubular Powder and Olin’s “Ball Powder” in US Contract loading.
Any “Air Service” Ammo used by ground forces was either “life expired” in its synchronised gun role, or, in the case of RAF ground units ( Airfield defence etc) Down-graded Air-type ammo after having been up in the Cold sky.