I've seen 'original' Winchesters that have had holes welded up and refinished, so well that I absolutely couldn't tell at all, at all. Please bear in mind that I had seen one of these rifles in particular (a prewar Winchester 61) when the holes were still there, and so I knew exactly where to look for the evidence.
Well, I'm extremely nearsighted and so can focus up close & personal, and I couldn't spot any evidence whatsoever. All the polishing marks ran the same way when inspected under a glass, there was absolutely no difference in either the color or the texture of the blue finish even at the edge of the redone area, and all the markings had been restored to match the originals (again when inspected under a glass). The craftsman had even gone so far as to include a broken section of the roll die marking to duplicate the original broken Winchester one, or else he actually possessed the old broken Winchester dies. I'm sure that X-rays would show the welds but my calibrated Mark One eyeball couldn't see any evidence at all.
The craftsman's identity was kept a deep dark secret by the person who handled the transaction. FYI the person who handled the transaction and actually acted as the middleman is the main operator of a well-known collector's auction house.
Caveat emptor, regards, Joe
