Little Machine Shop's DRO display - - voltage?

I have a 3-axis DRO display, labelled "Positron-3A", sold by the
Little Machine Shop. I don't have an AC adapter for it, and I'd like
to know if anyone out there knows the specifications of the adapter.
Since the unit takes 3 AA batteries, I'd assume that the adapter is
about 4.5v, but it would be reassuring if someone could confirm this,
and also tell me whether the center conductor is positive or negative.
Incidently, I have the user's manual, but it doesn't include adapter
specifications.