Value of grey stamp base set cards?

I found this at a local card shop and the stamp seems grey. I think I’ve heard that this is an error? Maybe someone has more info. It is in pretty good shape. If it is an error I may try and get it graded.

It can add a little bit of value to the right buyer for commons and uncommons, we’re talking a few dollars

Yea I didn’t think too much. It is technically an error?