This is a super rare and near perfect example of a Beattie Intenscrene (Model 83310)
This focusing screen greatly enhances brightness and center focusing especially when using a prism. Off-axis viewing will be noticeably darker due to this being a fresnel lens focusing screen. It is highly recommended to use the flip up magnifier or a viewfinder with this screen.
More information can be found at:
www.intenscreen.com