"With 10 major contractors plus the 100's of subcontractors providing various parts, there is no grand answer with which to grade by. There were 6 million "correct" carbines between 1942 and 1945. None of us can go back in time to the factory floor to see if the parts being put in this serial number carbine was from the top or the bottom of the parts bin"

This has answered my question.

That old carbine or garand that grandpa or uncle brought home from the war could have received treatment during the war... anything could have been replaced and there is no documentation to either prove or disprove this.