The 'IZ' is 'Mark 1 / Z'
Mark 1 the caliber and bullet weight/configuration of the cartridge.
The 'Z' designates it as loaded with a nitrocellulose propellant.
I wouldn't shoot them off. Not that they probably wouldn't work just fine and a bit of cleaning with soapy water or your other favorite cleaner for the job cures the corrosive nature of the ammo.
But a full clean box of Australian mfg WW2 pistol ammo has to have more value than that of just shooter stuff.
I think the box of 24 allowed for 1 load in the revolver and 3 full on 1/2 moon clips in the 3 segmented belt pouch.
FWIW, I've always thought that their (Australia mfg) 455 Webley ammo was designated .450 on the cartons.
Maybe just using up what they had during time of war,,and then I'm probably wrong!