Hi Rudolfo,
I am not an expert either, but from what I see, it would be difficult for an expert to tell if this is a legitimate Anotado due to the faintness of the image. However, I understand that the expert committee of MEPSI is sometimes capable of making a determination with even poor images. All I can say is that it appears to be about the right size.
The Follansbee catalog says that 6c Anotados are worth $650 to $675, so that is a pretty good incentive to have this stamp expertized. If it is real, no one will believe it without a certificate.
Best regards,
James