I used to sell a LOT of Mamiya when it was more available and stocked some repair parts. I am still turning up some odds like this guy. It is the top cover for tube style viewfinders for the Pro and TL. It does not fit the 645E (that has a dial on the side).
This is an easy install; just loosen four screws.
This unit is unused and in mint condition.
SKU: 210329-2