Thinking off the wall so to speak on this matter, I wonder if a decision was made, high up in the Ordnance or at the Maintenance Advisory Group (we call them 'the MAG's) a decision was made regarding Cadet Force No4 rifles. Maybe, after looking at this situation where a decision was made to the effect that
'.....if you think we're going to reject hundreds (and maybe thousands if it affected the No4 as well as the L39 and 42) of trigger guards because the loop for the now obsolete action cover is missing or formulating a repair programme to rectify the situation, then you're wrong. And if the sling twists on the swivel, then that's tough!'
A similar situation arose on several other occasions. One that I do remember was the bayonet standard on the Sterling Mk4/L2A3 sub machine gun. In the latter days, if this came loose or broke off (it was brazed inside tha casing.....) then it was simply removed, made good and that's how it stayed. Earlier, if this happened, you brazed it back or if it was lost, you scrapped the whole gun!.
That's just a thought about the work of the MAG's. But if they DID decree this, it never got to me