There are a number of decks that, using the additive form of Roman, use IIX instead of XII.
Though it may be considered an error in carving the woodcut, it is not one that impacts on the reading of the numeral.
I have used the additive form in a non standard way, for example, in both my cards for the
second and
third Aeclectic Tarot community projects (six as "IV" and sixteen as "VIX" respectively).
Our modern 'problem' with this is that we habitually use the subtractive form when using Roman numerals, and so tend to see, for example, 'IX' as 'ten minus one', rather than as 'one and ten' - ie, as 9 and not 11. This would not have been the case a few centuries ago.
