I have seen a number of receivers with these faded markings on the ring. Usually they are
starting on the top and fading to the right side. I have seen them on Winchesters only. I have seen them on 5M, 6M, and 7M guns (including my own). The late '44 & '45 serials. Prior to purchase I had mine inspected by an expert gunsmith/machinist/precision firearm manufacturer here in CT. His assessment was as follows (for mine) .... height of handguard ring lip uniform over entire top of receiver ring. "Chatter" marks evident from machining at edge of handguard/receiver ring. Uniform receiver ring curvature, no flat spots. His call was that the receiver was originally manufactured this way. He suspected the stamping tool was wearing out and needed adjustment, repair, or replacement. He said that it was an expensive part to replace even then. We know that Winchester had the oldest tools and machines at the start of production. Probably was wearing out. War was ending soon and the carbine was moving to Springfield Armory, so why bother doing anything more than apply a temporary fix to this problem whenever it cropped up in the late guns.