Ted,

I'm pretty sure it is a version of the PME51. According to a 1957 Hasselblad catalog I haven't had the heart to discard, it has the same design as the PM5 but has a center-weighted TTL meter with a silicon cell, ISO range 25-6400, scale illumination by a red LED, push button start and auto shut-off after 10s. Takes a 6V type PX28 or equivalent battery. Possibly it was pre-dated by as mentioned a PME45.

http://www.jeffreyglasser.com/