I have three PSA 8 ST and a PSA 7 ST 1964 Stand Ups, so to me the qualifiers were great (especially since the wax/gum residue is on the blank backs), because they reflected the 'actual' grades of the cards themselves.
Maybe reach out to PSA to see if they are still issuing stain qualifiers on those cards, or to SGC to ask how much a stain affects the number grade? Outside of that, I would imagine the (non-scientific) rule of thumb is to drop it two number grades from what it would otherwise deserve??
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