I have a Ferris Model 22-D Microvolter that is in almost pristine condition. I would like to use it for some work on my vacuum tube era SW receivers but do not have a manual. If anyone can provide information on the output impedance of this instrument and the operation of the zero adjust pot, I would appreciate it.

A microvolter will accept a fairly wide range of audio input. You normally adjust the meter to 0dB, then the output should be calibrated (if your controls and resistors are within spec). I don't know what impedance yours would be.

This is all I have:

That is a signal generator, not a microvolter.

Ferris called their generators "Microvolters" in the 1930s. Check out p.137 of "Tube Testers and Classic Electronic Test Gear."

Having a signal generator capable of producing a signal as low as 0.1 microvolt was a BIG deal at the time this was made. The typical stuff on the market then was poorly shielded and output attenuators were basically a joke. These generators were created in response to the developing need in the late 1930's for such a feature to service VHF FM and AM communications equipment where you need a way of proving that a receiver can actually hear down under a microvolt and where aligning an FM receiver required keeping the input signal below saturation of the limiters. Ferris didn't survive mainly due to some management issues and the marketing of the superior waveguide-beyond cutoff variable attenuator by Measurements Corp. in their portable generators such as the Model 80.