Posted By:
HankronFirstly and except in blatant cases, I shy away from making a definitive opinion based on the moire pattern in an online image, as modern scanners can do all sorts of funky things. I find it essentiall, if not always possible, to view an item in person before making a final decision.
It has correctly been stated moire patterns are created when making a scan of a card made up of the half-tone dot pattern (ala all those tiny dots in a 1957 Topps or Exhibit card). Lack of a moire pattern doesn't prove anything, as many 1957 Topps or such have been scanned without such a pattern. But if you see such a moire pattern on a Harper's Woodcut or Old Judge or Fatima card, it can be reasonably assumed that the card is a reprint.
I have not seen a T3 in person in some time, and I don't know how they images were made (dots versus no dots). If they don't have the dot pattern, then it would seem unlikely that a moire pattern would appear in a scan of a genuine card.
Note on T206s and related E cards. The half-tone was used differently for these cards, and the half-tone dots (and resulting moire pattern) in the Portrait cards usually appears only in part on the image. So if you see a T206 Portrait and there is moire pattern in the face (for example) but not in the background colors and white borders, this is evidence that the card is gentuine.