What is the purpose of the Ejector Screw?

[Alan de Enfield]

It only comes into play if you have an FTF.
As hayboy says - the "drag" of the case rim on the inside of the body is insufficient to flick the weight of an unfired cartridge out. The ejector screw is specifically located so that the case rim reaches the tip of the screw just as the bullet nose on a MkVII round (with an overall length of 3") clears the chamber
If fitting a PH sight using the ejector screw hole ensure that you buy the correct replacement screw that is long enough to go thru the PH sight mount, thru the body sidewall and just protrude enough to do its job.