Thinking on my feet, I wonder if the original Bren mag boxes in Aust service were converted to 7.62mm spec and retained for use with the L4's and the L2 rifles.

Remembered now why the modification was slightly different in Oz. The UKicon L4 mags were slightly curved but the Oz L4/L2 rifle mags were straighter. Seem to be as reliable as each other although we'd occasionally see curved pommy magazines