Thank you for the other answers and thoughts about this grill. In the meantime I tried a scan where I didn't see anything at all and several other photos of which I show two here. Actually I see this grill pretty clear and normal with my eyes but he hides quite skillfully when it comes to getting captured on a non volatile storage media.
By the way I decided to not use graphite - as I want to keep a Z grill which is difficult as an example for the future. If it's more clear it's not a good example for the next stamps in question.
I know I already got some answers but I am glad to get more opinions if these news photos show more. The longer I look the more nice horizontal ridges I discover.
Also I would like to know: If it could be a Z grill but is damaged (to 1/20 of CV as mentioned here before) I wonder if you in my position would trying getting a certificate if the stamp is just worth the fee?

