I touched up the photo for him and used some tools in Photoshop to enhance the grill. Hope these help. I count 14 x 16 points.
Another thing you can do is to take a #2 pencil and color in a black spot on a piece of paper. Take your finger, and lightly rub it on this spot. Now, take you finder, and very lightly rub it on the back of the stamp on the area of the grill. The grill will pop out at you.
Since the stamp is used and has no gum, doing this should not affect the value.
Original with color correction
Enhance 1
Enhance 2
Grid