Mamiya RZ67 Instructions (1987 Version)
Page 46, page heading
Accessories
Focusing Screens RZ
Type B Rangefinder Spot
Same as Type A Matte, but with central split-image rangefinder wedge added. Matte surface can be used for focusing, but split-image rangefinder significantly boosts focusing precision."
The RZ67 manual on various websites now is, I think, a different version that doesnt list the Type B focusing screen. The type A screen is the standard screen installed on new RZ67 cameras.
My manual also has line drawings accompanying each screen described in the list. The drawing matches the description.
From your description of the screen in the box, it is most likely a standard Type A screen in a box that originally held a Type B screen.
The type B screen is distinctly different from the Type A screen in having the central split-image rangefinder.