Digital Phasemeter - what is it?

[AdamAnt316]

Hello, everyone. At a recent hamfest, I spotted an Acton Laboratories model 330A "Digital Phasemeter". I mainly bought it for the nixie tubes it uses for a display (four, plus one for the "-" negative symbol), but I'd at least like to find out what it's used for, and if I might have any sort of use for it before I go about stripping it of parts. Unfortunately, information on this sort of piece of test equipment seems to be fairly thin on the ground, unless it's better known by a different name. Any ideas? Thanks in advance!
-Adam