How was the image transferred? To SD or HD? If SD, was it transferred as anamorphic 16×9?
Your assumption that a 1.66:1 film needs a 16×9 mask is incorrect, as 16×9 equals 1.78:1 not 1.66:1. In the effects palette, under Film, you should find a 1.66:1 mask. This mask only works correctly in 4×3 SD projects, though.