I am classifying this stamp perf 12:
The truth is that it is difficult to see with the naked eye but with some light a grill can be observed.
The previous owner wrote 29A behind the stamp but I think he was wrong. It seems F grill to me if it is a real grill.
What is your opinion about grill? Which Scott number could be?
If it can helps, the feel of the paper in hands is very thin.
Thank you very much to all