Once we actively were engaged in hostilities this marking was no longer necessary and the markings were dropped as a unneeded step in production.
Um, not really, see pictures. This is a late 1943 No4Mk1*
the screw you are referring to is the ejector. when you retract the bolt after firing a round the extractor hooks onto the far right portion of the case rim and as the bolt travels back the rim catches on the tip of that screw protruding through the reciever, flipping the shell out and allowing the extractor to disengage the casing.
One of the odder aspects of the Lee-Enfield design is that the ejector screw has noting to do with ejecting fired cases! In fact fired
cases are ejected by tension & friction against the left side reciever wall applied by the extractor spring. (That's why so many problems are cured by replacing a weak or broken one). Trust me, try it with a fired case while looking into the top it will eject long before it reaches the ejector screw.
The screw actually ejects only UNFIRED rounds.
Regarding the "L" flip sight.
The settings are indeed 300/600, but the 300 only works correctly with a bayonet fitted, without it is 400. SOP at the time the flip sight was fitted was to fix bayonets if the enemy was under 300 yds so it was set up this way.