[It seems as though over the years and tears, nobody had noticed that a few of the guns had been issued with these original magazines.

Peter, then the 'Circuit Armourer' who did the rounds with establishments that did not have it's own Armourer. Wants shooting for failure to carry out his duties correctly!
We both know that one of the many onerous tasks was the inspection of magazines & appurtences! Boring, boring, but it really did need doing. Some mags got damaged during usage as you ar familiar with, im sure.
These early L4A1 mags SHOULD have been weeded out during this procedure. But as your also aware, TA & Reservist units DO have some quirks & 'Interesting Things' hidden away from normal prying eyes!..........LOL!