Sometimes a mark on a card looks like it's post production but it actually isn't

[Pat R]

When I first looked at this Mitchell card I thought at some point it had
been marked with a black marker.

Mitchell.jpg

But after a little research it turns out the mark occurred during
the printing process.

Mitchell With Black Mark.jpg

SGC doesn't put MK qualifiers on the their cards but if this card was sent
to PSA for a crossover it would most likely receive a MK qualifier and
rightfully so. Even under magnification you can't tell that it occurred
during the printing process.

[Pat R]

Here's another pair that have matching marks on two different
cards.

Ball Mark.jpg

[Leon]

I agree it might get an MK but of course you are correct. Lots of those examples out there and they probably go both ways. But most times I would think a TPG won't do the legwork and just slap MK on the flip or lower the grade as it is one.