This was when those in charge knew for a fact that common soldiers will waste ammunition, therefore they had to be restricted to single shots only, until such time an officer ordered them to use the magazines. Of course such thinking dated to the time when single shot military rifles and volley firing were the norm. It took time and a big war to change the thinking, but once the production model is fixed it is often too expensive to change, or the thinking hasn't and the expectation of the next war would need only single shots.
With the Lee Enfield it was quite simple to stop the machining for the cut-off.