When Arthur empties the revolver that was just fired at him none of the cartridge cases are empty.
Coming of age in the UK was only reduced from 21 to 18 in 1970, so Polly's 17 year old son could not come of age at 18, but would have had to wait another 3 and a fraction years to run away from home.
...28 min, after a shot was fired, Arthur grabs the Webley revolver from the grieving mother and unloads it on the table. All 6 six cartridges have lead bullets (slugs) instead of 5 cartridges and one empty case.
Michael comes of age at 18, but the age of majority at that time was 21, and only became 18 in 1970.