Quote:
I wonder what grill may be?
As orstampman notes, this is a normal G grill.
The production error was a double foldover in the paper after printing. This results (when separated with scissors between the stamp images by the end user) in half the perforations lacking. As for the unusual grill impression, the grill was pressed only
once. Consider this model of the back of your stamp. The points in the diagonal strip "B" are opposite those in area "A" and "C". The grill runs off the edge of your piece and is not complete.

I see no reason to get a certificate. The serious students of this issue would know the cause and effect.