So the 1c Z grill is only an EFO by that reasoning.
Apples and oranges. Consider the context of my remark, i.e. determining whether the pink paper was intentionally treated thus. How is that different from the way the Z grill came about and its use on various denominations? I see a significant difference. But let us not forget that there are students of the matter who have suggested that the 1c Z-grill should be regarded as a trial piece. Ditto the B-grill.
What it boils down to is the question of intentional issuance.
Was the pink paper intentionally issued for some purpose? If so, why so few? (As in - no known certifiable examples.)