This only works with the Late Mamiya Universal Body. It goes on like the M adapter with the slide locks.
You remove the M adaper with whatever back you're using, including Polaroid, and you drop this in place for fine focus. Then you remove it and replace your M adapter with back and shoot.
It has lines for 6X7, 6X9, and 3X4 Pola. The leather hood and door come off with the 2 sliders for those who prefer a serious focus hood.
This is super late issue like 1980++. I've only seen a couple of them. They are scarce. $125 + shipping. Complete your outfit.