A friend of mine picked this up at a garage sale where an elderly electrical engineer was selling a bunch of old test equipment.
It's missing a faceplate where it should name what it is, and he's googled the company name and model number, all to no avail.
Several of the engineers in my company have been by to take a look at it, and so far the consensus seems to be that it's some sort of tunable filter (it has what appears to be a large variable capacitor and large variable resistor).
I was wondering if anybody here had seen one of these before and had any idea of what it was used for, and possibly some links to relevant documentation.
UPDATE: Added image of interior of device