Optical illusion, Don. Incidentally (chuckle chuckle), this is why it is far more valuable to learn how to shed light onto your grill specimen than it is to soil it with graphite, which your site of higher philatelic learning STILL promotes, against the advice of the most advanced students in this community.
Edit:. The stamp with the partial straight edge shows an F-grill.
The background has not changed, it is the direction of the light relative to the background that is different. This is why I recommend to not only cast light onto a grill at a low angle of incidence but to also be sure that it is not coming from a direction that is parallel or perpendicular to the rows and columns of the grill.
The light source should be low and shine from corner to corner. Very simple. Very effective. Harmless.
Hi Winston, The Stamp Smarter grill article states, "While using graphite has been widely accepted over the years, there are some who feel this is not a good practice. They believe it alters the stamp and that grills can be viewed without graphite using methods such as watermark fluid." Since this is stated in the article, I do not view this as "STILL promoting" the use of graphite.
But it is true that the site could use more information on the alternative methods for grill detection and identification. If you willing to write an article on this topic to help communicate these alternative methods I would be happy to make sure it gets published. Alternatively email me just a short paragraph or two and I can add it to the existing article (this would not be much more effort than writing your post above) and I will add it as a side bar and attribute you as the author. Don