"...Is this something..." It's Hollywood and/or a broken or worn sear. The M1 Rifle was not designed to function in FA mode of any kind. Mind you, a carefully placed bit of wire will make one rock and roll.
There have been modifications to put a 20 round mag on an M1 though.
"...pull the offenders trigger housing..." Basically disengages proper functioning of the sear. The troopies have always been a clever bunch and knew how to re-work things. That's how Bradley got 1stArmy through the bocage. A sergeant came up with the idea to put teeth on a tank. Did it with German steel too.
"...credit to the Canadian designer..." Sort of. JC's first design had a 20 round box mag. The U.S. Ordnance Dept. said they didn't want it.