Before the PME51, the 45s metered viewing screens were, PME5 and PME3. They are calibrated for the 'acute-mat' screen, the readout is in EV values. They are also powered by a six volt PX 28 battery.
Going back to the first meter prism, the exposure was indicated by a swing needle which which it is said, is not suitable for modern photographic techniques, not my view but Hasselblad!
Next came the PME/VC6 prism finder which was designed for the old type focussing screens.
From your photograph, I am unable to be more specific, but it must be one of those mentioned as there are no other meter prism finders prior to the PME51 that were produced by Hasselblad. Stan. L-B