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boxingcardmanOne (1953 Spahn) came back evidence of alterations by PSA, and I agreed. From a CSA 7 down to garbage. The others crossed over a grade lower (T206 Waddell) with SGC, and the last one came back same grade (1954 Topps Williams) from PSA. It's been years since I did this, however--I got the cards from Teletrade, which gives you some idea of how long ago...
FWIW, I do not believe you can ever get a truly unbiased opinion from any grading service on someone else's holdered card.