Lee this screw is often seen on early LJ's and was probably a factory upgrade. So you're right, it wouldn't have been on the rifle when it was new but was almost contemporary with it and shouldn't affect the value too much IMO.
IIRC, the idea with this 'innovation' of LJ's was that wear in the tap would be taken up by tightening it - but the idea was dropped when it turned out that doing so pulled the tap out of alignment with the bore. Obvious when you think of it!