Looks to be a PME-45;
http://www.bhphotovideo.com/c/produc...der_PME45.html

Hasselbladhistorical has a manual for the PME90, which I think, has similar functionality.
here; http://www.hasselbladhistorical.eu/HW/HWVSys.aspx

In general, I like the 45 deg. prisms a lot, I have a PME51 which is an older metered "PM" type prism.