Can I legally mark the center of the club face (irons)?

I kinda liked the circle in the face of my old Cobra irons...

I'm curious, does it make a club non-conforming to mark the sweet spot say... with a sharpie? Draw a line down the middle of the face, or a circle? What about engraving or stamping?

I read the equipment rules regarding the club face and it does allow for "decorative markings" at the center of the face, however I don't know if I am allowed to modify the club myself, or if the sharpie mark counts in this regard.

Thanks in advance. I've googled and googled and come up with nothing.

it is fine as long as the intent is not to influence the movement of the ball