I doesn't fit onto the rotating adaptor. You take that off and then the polaroid back goes on there, directly. At least that's how my two polaroid backs works- they don't rotate.
I still (happily) shoot type 665 and 55 in the rb. It's wonderful. And fuji 3.25" pack stuff too. I have the medium-format (pack) polaroid back and the 4x5 (loose sheet) back; with the latter, you can also shoot fuji quickloads on the rb. As I recall, neither goes onto the rotating adapter, that has to come off.