Posted By:
Jeff SIt seems like MK is a useful idea, especially for particularly rare vintage cards that have been collected for a long time--like old judges with "N172" pencilled in on the back, or nice cards with a previous owner's stamp.
It is useful in the sense that large numbers of very attractive cards (like the one Dan is talking about above) would be automatic "1's" because of a pen mark or a stamp that may not mar anything other than an unoccupied corner of the back.
If consistently applied, the MK designation also seems like a good marketing tactic for PSA. I have lots of cards like those described above -- would be "ex" but for a stamp or something. Those cards rarely get authenticated and encased because their owners assume they'll be 1's.