mdroth thanks for your honest opinion. The risk is always there that some forgers manipulate the stamps with inks etc. I dont think its glue on the grill its only reflection from the light. But I think its genunine withouth manipulations.
The apparent number of grill columns puts it well outside of the F-grill range and seems to be right for an E-grill. Couple that with the color and it is in likelihood an E-GRILL, Scott #88. Better images would help confirm.
I dont jump on the bandwagon. Its not an F-Grill and E-Grill, this stamp has more horizontal and vertical points than the mentioned. I agree with Ok1 that its a point up Grill. This is to asses when the ink is on the peaks. And this stamp fulfills this. This grill was put on the backside of the stamp so that there are peaks in front of the stamp that are raised.
Roughly the grill seems to go where the yellow lines are. I dont whant spend hours to determine this. Its far from accurate but the grill seems relatively big to others. It would be better to determine with a proper scan.