The M19 was replaced by the M19A1 around 1954, when the shorter 7.62mm NATO cartridge was adopted. I had read that one of the improvements was making it narrower by about 12mm (63mm to 51mm) because it had to hold the thicker (necked down) 7.62mm NATO without it rattling around.
The M19 was also supposed to carry .30 Caliber [7.62x63mm] 250 rounds belted / 225 rounds linked and the M19A1 holds 200 rounds 7.62mm NATO [7.62x51mm]; I also assumed it was shorter as well. I’d like to see them measured up against an original M19 ammo can for comparison.
I’m surprised to see Australia using M19A1 cans rather than British H.82 .30-caliber cans. It may be due to NATO standardization / compatibility or closer military ties to the US than to the UK.