To straighten out a misconception, the "MG" in the printed section of the bandolier is for the Footscray Ammunition Factory No.2 which came into being in 1940 and this identifier was used through until 1949 and reverted to MF. It does not indicate Machine Gun ammunition which seems to crop up repeatedly on forums etc.
The rectangle with the line through the centre indicates Cartridge Small Arm Ball and is present on packaging of 303 Marks II,IV, V, VI, VII and VIIZ ammunition and was used on bundles, bandoliers and 48 round boxes and other packaging e.g. 1248 round cases. Not having seen any Mark I cordite bundles or early Mark VIIIz packets although I presume it was used. Australia used the symbol particularly on .303 Inch bandoliers until 1960.