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I've opened well over 300 (that's when I stopped bothering to keep count) and have yet to get a single mask so you just need to be extremely lucky I suppose.
I have received three of each, on top of the ones that I 'learned' although to be fair I was farming all of the Dolmens, Delves and World Bosses in Tamriel and earned a high number of plunder skulls from doing this.
"Such is the nature of evil. Out there in the vast ignorance of the world, it festers and spreads. A shadow that grows in the dark. A sleepless malice as black as the oncoming wall of night. So it ever was, so will it always be. In time all foul things come forth." -Thranduil
the drop rate is really really low, I am also farming it, but did not receive it yet, but probably yes, today still, the witches festival ends 1. november, 10AM EDT, which is 3PM UCT+1 (Wien, Bratislava, Prague - my time zone)